How to get the maximum out of your watermelon production

Some growers can reach yield of 100 tons per hectare and for many average is 50.Yield of 20-25 tons does not make profit. Financial inputs and efforts in intensive productions are very high and every mistake relates to lower quality and yield as well. It is very important to find a balance between input and output. With up to date techniques of drip fertigation, mulching, small tunnels and grafting many growers manage to expand harvest period and make very profitable productions. 

Origin

Watermelons (Citrulus vulgaris, sin.Citrulus lanatus Thumb.) originate from hot area of central and south Africa so the species itself is very sensitive to low temperatures and light intensity. They belong to Cucurbits family where the cucumbers, sweet melons, squashes and pumpkins also belong. All of those species have similar production demands meaning that the best growth is on rich structural soils with lot of added manure. By taking into account that they are susceptible to same pests growing on the same field or rotating watermelons with mentioned cultures from the family should be avoided for 5-6 years.

Climate condition

On temperature of 25º C seed germinates after 4-8 days and this temperature is optimal for the start of plant growth as well. Young plants could not survive temperature of 1º C, growth stops at 10º C and minimal temperature for growth is 18º C. Plants do not produce flowers on temperatures lower than 13º C. Mature plants can survive on high temperature of up to 45º C. During vegetation period in order to produce fruits they need 1.200 hours of sun light and temperature sum is 2.000-3.000 º C. That means that we can harvest watermelon fruits around 20th of June from young plants that are planted beginning of May. Average vegetation period of Vasko in south east Europe is 60-65 days from planting to harvest, Varda harvest starts after 70-75 days while Kodak needs 80 days. If the plants collect this sum earlier, the vegetation period is shorter and yield is lower. Lower temperatures affect poor fruit setting and sugar content is lower than 12%. Insufficient light intensity results in smaller yield and poor fruit quality. Light is essential during the flowering and fruit growth period while the species itself is neutral to the day light period. Watermelons are not sensitive to low air humidity while high relative air humidity (RH) could effect pollination and poor fruit setting. Also plants are more sensitive to fungal diseases under high humidity. Although watermelon can reduce stressful conditions of insufficient water in the soil for the short time period available water in the root zone is important in the period of intensive plant growth, flowering and fruit development. Watermelon production can be both successful from the production by planting young plants from the nursery or by direct drilling in the open field. Both of those production techniques have its advantages and disadvantages while using young plants justifies its purpose in early production.

Young plant raise

Purpose of this kind of production is to begin with harvest 10-15 days earlier than usual. Sowing starts in first week of April. If small tunnels will be used than we sow as early as 20th of March. For later harvest period start of sowing is until end of April. With temperature amplitudes of 7º C at night and 25º c during the day germination could take 10-12 days. If it’s manageable to obtain stabile temperature of 25º germination starts 4-8 days after sowing. Very common mistake is to start production in nursery without heating facilities. Low temperatures together with water soaked substrate prolong germination. Even if seedling starts growing toward surface very often young plant is not capable to raise first leaves above the ground. If leaves do appear undeveloped root system is not capable to feed young plant.  When more favorable climate conditions appear later during production plants suddenly start falling. That is physiological symptom. Leaves evaporate more water than root is able to provide. Consider root as a “mouth of the plant”.

In less than perfect germination environment it is very important that seed has high germination energy. It also provides uniformity of plants. Seed size varies among  varieties. Some growers have a habit to moist the seeds by dipping it in water that has a temperature of 32-35º C during 2-3 hours. Sowing starts when 5-10% start germinating. This technique is not recommended because the protective layer of fungicides on seed (Thyram,Captan,Mankogal) can be washed off. In order to prevent “damping off” the  first treatment with fungicides has to start immediately. Seedling “Damping off” can be caused by 4 various fungi (Pythium is most common) and can be prevented by an early treatment with Previcur.

As for cold temperature conditions in April it is important to have proper air/water ratio in root zone. Substrates based on peat are a much better environment for the root growth than self-made mixtures. In sterile peat mixtures (by Klassman, Flora Gard, Stender, etc.) during stressful conditions plants will survive for longer period. They are also disinfected so plants are less sensitive to diseases. Many of those substrates have all necessary nutrients for the first two weeks of plant growth.  Only disadvantage of using peat substrates is that constant care of humidity has to be taken because water will evaporate more rapid than it would be usual by using self made substrates based on manure and garden soil. There are different recipes for substrate preparation. If chicken manure is used than it should take less than 20% of the mixture. Higher percentage would be harmful to the root. Standard recipe is one third of compost, one third of rich garden soil and one third of peat or fermented manure. Instead of peat sand could be used as well. In the mixture 3-5 kg of NPK mineral fertilizer (15:15:15) per cubic meter should be added. Problem is that self made mixtures have to be disinfected, usually with strong fumigants (Basamid, methyl-bromide). Mentioned chemicals are only effective at 15º C air temperature and between this treatment and usage of substrate we have to wait for 10-15 days so chemical compound will destroy germs and living organisms. Due to emission of poisonous gases those chemicals already are or will be banned in many countries. Utmost care of a skilled person must be taken to do the treatment strictly according to specification provided by manufacturer.

Optimal depth to place the seed is 3-4 cm. After first two dicotyledon leaves appear plants begin with intensive growth. In that period after initial 25º C lower night air temperature to 14-16º. During the day if it’s cloudy optimal temperature is 15-18º C and if it’s sunny 20-23º C. Reason behind it is to stop plant stem elongation and make more compact plant habit. Relative air humidity  (RH) should be 70-80%. Very important is to have facilities to quickly ventilate nursery. Ventilation facilities must be capable to adjust temperature but air humidity as well. High air humidity is good environment for fast spreading of many fungal diseases.

Good, high quality young plants are foundation of success in further production. All further stages of plant development have their history written during young plant production, meaning it is not possible to have a high yield and good fruit quality from poorly produced young plants. Besides health status while evaluating quality we should look for strong root system. During young plant production constantly watch root growth progress. If enough space for each plant is not provided root system will go outside the pot or substrate block and if let in the air will go dry, harden and stop with further growth terminally. Dried part of the rot system will not be functional anymore. In order to acquire and preserve strong root 10 cm (minimum 8) diameter pots should be used. Many growers instead of pots and blocks (10 x 10 cm) use plastic bags filled with substrate. Than at least 1 liter of substrate should be in the bag.  Root system of different plants while growing in the adjacent substrate blocks could mix and interfere with each other. Separation will cause breaking and open wound. After planting in open field soil pathogens could enter through the wound with more ease. If the root system is damaged than adaptation period after planting will last 2-5 days longer. Solution to the problem is that if substrate blocks have to be separated than it has to be done one week before planting. Immediate growth start of transplanted young plants is very important. Prices of fruits can drop within a week if harvest is delayed. For the same reason plants have to be well prepared for new outdoor (more harsh) climate conditions. Process is called hardening and procedure is that tunnels are uncovered in day period during the week prior to planting. Also it would be useful to do one foliar treatment with 0,5% solution of potassium sulphate. After this treatment rinse leaves with clean water! In order to prevent “ dumping off” treat young plants with fungicides regularly. Chemicals such as Cineb (0,3%),Previcur N (0,2%), Ridomil MZ (0,4%) and Benlate (0,1%) are effective. Also do one treatment against insects (Actara or Confidor). After the first fungicide treatment in dicotyl stage next treatments are every 10-14 days. Same active ingredient could be used maximum two times.

Many growers do first foliar feeding of plants at the stage of first or second true leaf. Second foliar feeding goes after 10-14 days. It is important to take care of concentration of fertilizer. Foliar treatment solution has to be less concentrated than solution used for feeding through drip irrigation system. So right measure is 0,02% instead of 0,4-0,7 % (in drip water). Take attention that high concentrated solution could do more damage than benefit. Some science papers indicate 5% increase in yield if 3-4 foliar treatments are done during vegetation. Some growers use mineral NPK fertilizer (2-5 gr. diluted in 10 lit. of water). In that case after the treatment rinse leaves with clean water.
Most common way of irrigation in grower’s nurseries is sprinkling above the plants. During the sunny weather drops of water are acting like small lenses so sun beams can hurt sensitive leaf surface.  To prevent formation of big drops use finest micro sprinkler at the end of the hose.

During the sunny weather “glasshouse effect” can considerably raise temperature in tunnels. Constant temperature control is necessity. If fresh manure is used as a heating source (and in many regions still is) it adds up to cumulative heat effect. In non-ventilated tunnels it is possible to reach temperature peak as high as 60º C. Poisonous gases from manure fermentation are present as well. In practice same number of production failed for the reason of too high temperatures as of low temperatures. Heated pipes (hot water) are the best for temperature control in plant beds.

Common mistake is to irrigate with cold water as it’s a considerably stressful for the plants. Keep temperature at 18-20º C. This is not easy to accomplish even if we are using ponds. While making construction plans for the nursery always take into account that the water pond (tank) will be located in the warmer and closed space.

Quantity of water is 5-10 liter per m2. Too much water is harmful as the lack of it. The root needs air as well. In cold period irrigation frequency is every 5-7 days and as temperature rises irrigate every second day or on a daily basis especially while using pure peat moss. Few days before planting quantity of water should be reduced. In the evening prior to planting irrigation should be done in order to separate plants more easily.
 Take care that root system with its surrounding soil is preserved during movement of plants from the nursery to the open field. Strong wind or sun can dry up the roots. Good nourished plants should be 25-35 days old. Appropriate planting date is when there is no danger of frosts. Usually it’s beginning of May, even 20th of April if small tunnels are used to cover the plants. Field should be prepared few days earlier. If we are using mulching foil (20 micron, 1,2 m wide) than placing it through the field should be 10 days before planting. For early production transparent foil is better than common black but field have to be absolutely spotless from weeds. Green foliage is intermediate solution and gives best results. Some growers put amount of one soup spoon of fertilizer in the hole before planting but take care that root will not be in contact with fertilizer, so on top of the fertilizer generous amount of mud should be put in the hole. Mud would help root to accommodate immediately as well. This practice is proven and strongly recommended.

Direct sowing

For mid-late and late production often there is no need to start production with young plants. Direct sowing gives the same results and is considerably less expensive. In any case decision on appropriate production field is important. Watermelon and sweet melon are very sensitive on herbicide chemical residue compound based on atrazine. Check the history of chemical treatments of designated field. Take care on crop rotation. Good choice would be the fields where wheat (all Poaceae family) or legumes were grown (peas, beans, French beans). By all means avoid cucurbits in the first place but also family of Solanaceae (pepper, tomato, potato). Many growers emphasize importance of deep plowing in the autumn. They plow as deep as 30-40 cm during October or beginning of November. Before plowing 40-50 tons fresh or fermented cattle manure are dispersed on the surface. It is also useful to do one additional 15 cm plow during the spring. Take care that if manure is not fermented than it could only be added in the autumn. Fresh manure added in the spring can seriously harm the plants as for spread of diseases, insects and weeds. Also take utmost care that for disinfection in stables chemicals based on Cl (chlorine) were not used. Watermelons are very sensitive to chlorine. In spring we close open rows and do cultivation. If manure is not added in the autumn than smaller quantity of 20-30 tons of well fermented manure could be added in the spring. Sowing preparation consist of 1-2 cultivations when some pesticide against soil borne insects should be added (e.g. Force 1 lit/ha). Wireworms could make big damages but various caterpillars could be dangerous as well. Careful monitoring and identifying larvae and adult stages can save a lot of trouble. Not only they destroy young plants but drip irrigation tapes as well! Some grower still use insecticide Lindan which is not allowed for use in E.C. Lindan affects taste of watermelons and could cause digestive problems to consumers. The best solution to the possible problem is to consult plan protection professional. In watermelons there is very  limited number of selective herbicides that could be used effectively. Against weeds before germination or planting Roundup, Reglone or Basta could be used. Applied between mulch foliage Dual could be effective. After germination Fusilade, Select, Focus Ultra, Targa Super, Furore Super or Panthera could be the choice. Watermelons could also be attacked by very small insects (e.g. aphids, red spider). They are not visible without magnifying glasses. Regular visits by crop protection professionals could stop further spread that often starts from the edge of the field. Good control results are with insecticides such as Demitan, Talstar or Actara.
Watermelons are also susceptible to various diseases such as Fusarium oxysporum, Colletotrichum lagenarium, Alternaria altrenata, Erysiphe cichoracearum, Sphaerotheca fuliginea and Dydimella brioniae.

Healthy plants till the end of harvest are enabling high yield. Experts usually recommend a 7 day schedule pesticide treatments. Some of diseases can be treated in developed stage but some need strong chemicals that will not decompose till harvest. Most of fruit size is developed in just two weeks!

Damping-off

Damping-off is caused by a seedling disease complex that usually involves Phythium spp., Rhizoctonia spp. or Fusarium spp. The amount of damping-off is usually directly related to litter from the previous crop and to environmental conditions. In some years, seedling diseases reduce stands by 50 percent; in other years, seedling diseases are rare. Good cultural practices and seed treatment are essential in preventing damping-off of young watermelon seedlings. Basically, conditions unfavorable for rapid emergence, which involves cool, wet weather, are usually most favorable for damping-off.
Gummy Stem Blight
Gummy stem blight is caused by the fungus Didymella bryoniae.. This fungus can cause damping-off, crown rot, leaf spot, stem canker and fruit rot of watermelons. Lesions on the cotyledons and leaves are round or irregular and brown in color. Lesions on the crown and stem are brown and usually turn white with age. Early infection usually comes from diseased seeds. On older leaves, brown to black spots develop between the leaf veins. The first spots usually occur in the lobes of the leaves. Gummy stem blight spreads from the center of the plant outward. As the season advances, gummy stem blight attacks vines, causing elongated, water-soaked areas that become light brown to gray. Vine cankers are most common near the crown of the plant. Gum oozes from stem cracks, and runners usually die one at a time. In fruits disease first attack inner structure and lately appears on the surface when gum oozed spots appear. Infection goes from plant residues but also infected seed can start it. Optimal temperature is 20° combined with high humidity. Two year crop rotation is necessary. To breed resistant hybrids is very difficult. Satisfactory protection can be done with Mancogal, Ditan, Kaptan, Venturin, Cineb, Propinebor by systemics such as Befugin, Benomyl, Benlate. 
Anthracnose
Anthracnose, caused by the fungus Glomerella cingulata var. orbiculare (Colletotrichum lagenarium, C. orbiculare), can be a destructive disease of watermelons. This fungus attacks all above ground parts of the watermelon plant. Optimal conditions for disease are high soil and air humidity combined with temperatures of 19-24° C. Plants can be infected at any stage of growth; disease symptoms are first noticed as round to angular reddish brown spots on the oldest leaves. Spots may later dry, turn almost black and tear out, giving the leaf a ragged appearance. Often the leaves at the center of the plant die first, leaving the stem and a portion of the runners bare. Light brown to black elongated streaks develop on the stems and petioles. After a few days of warm, rainy weather, every leaf in an entire field may be killed, giving the field a burned-over appearance.

Round, sunken spots may appear on the fruit. Spots first appear water-soaked, then turn a dark green to brown color. The pinkish-colored ooze often noticed in the center of the sunken spot is spores of the fungus. Use treated seed and do preventive treatments with fungicide with active ingredient based on mankozeb (Dithane M-45), cineb (Cineb S 65), maneb or kaptafol every 7 days if the weather is humid (rainy).
Alternaria
Black spot is caused by fungus Alternaria f.sp.cucurbitae. There is no strong resistance among varieties against it. Appears if cucurbits are grown in monoculture. Presence of insect Tetranychus turkestani can spread disease faster. Main symptoms are spots on the older leaves close to the center of the vine. First spots are small and round and later they reach 2 cm diameter with concentric circles. At the end they are black and leaves become dry. Pathogen can survive 1-2 years on plant residues. Humid conditions are favorable for spread. Most susceptible are young and old plants. Preventive treatments with active ingredient mankozeb, captafol,chlortalonyl and maneb could stop further development.  
Fusarium Wilt
The fungus Fusarium oxysporum f.sp. niveum causes Fusarium wilt of watermelons. Fusarium wilt is widespread in many fields. Symptoms can occur at any stage of growth. Infected plants develop wilt symptoms on one or more runners, usually beginning at their tips. The vascular tissue in the lower stem and roots develops a light brown discoloration. In severe cases, the entire root may become dark brown and a soft rot develops near the crown. The pathogen can spread to new areas on seed or in soil transported by equipment, drainage water and man.Disease is more likely to occur on nitrogen rich humid sandy soils where pH is 5,5-6,5.  Several varieties are considered somewhat resistant to this disease. However, even with resistant varieties it is desirable to use new land or have a minimum of eight years between diseased crops on the same land. On old land, some wilting can occur even with resistant varieties; final thinning should be delayed as long as possible to eliminate the great number of wilt-susceptible plants before the final stand is established. Contamination of new fields with soil from Fusarium infested fields should be avoided. Using fungicides usually does not give satisfactory results. Grafting watermelons is good solution if crop rotation is not feasible.
Powdery Mildew
Often two fungus are the cause: Erysiphe cichoracearum and Sphaeroteca fuliginea. Firs symptoms are in inner side of leaves where white powdery smears occur. Attacked leaves dry and deteriorate. Fruits are not directly attacked but since there is no protection of sunburns they also affect quality. Optimal temperature for infection is 25° but it starts even at 15° and could continue at temperatures up to 30°. It is important to remove plant material of cucurbit family prior to planting and grow resistant varieties as well.fungicides based on  Karatan, Fenarimol, Hexakonazol, Miklobutanil, Triadimenol, Triadimefon, Tridemorf and Triforin can stp disease. 
Downy Mildew
Downy mildew is caused by the fungus Pseudoperonospora cubensis. This fungus attacks only the leaves of watermelons. Lesions first appear on the oldest crown leaves as yellow, mottled spots with indefinite borders blending gradually into healthy portions of the leaf. Older lesions are dark brown with a slight yellow border. As the disease progresses, brown areas coalesce, causing leaves to curl inward toward midribs. Under favorable conditions for disease development, downy mildew develops rapidly, resulting in a scorched appearance over an entire field. Optimum temperature is 16-22° C with high humidity. Chemical treatments with funcides based on azoxystrobin (Quadris), Cu hydroxide (Blauvit), dimetomorf + mankozeb (Acrobat MZ), chlortalonyl (Dakogal), metalaxyl+Cu oxychloride (Ridomil) and metalaxyl+cineb (Ridomil Z) are effective.
Watermelon Mosaic Virus
Watermelon mosaic virus I and II are now known as papaya ringspot virus-watermelon type (PRSV-W) and watermelon mosaic virus (WMV), respectively. Several other viruses affect watermelon; all have similar symptoms. The most common symptom is mottling of the leaf. However, mottling may be difficult to see under certain weather conditions. Some plants are stunted with abnormal leaf shapes, shortened internodes and bushy erect growth habits of some runner tips. The first symptom on the fruits is usually a bumpy and mottled appearance of the fruit surface. This disease symptom is strongly expressed in periods of extended high temperatures, which occur just before watermelon harvests. These viruses are spread by aphids, which can spread through an entire planting during the growing season
Rind Necrosis
The cause of rind necrosis is not known. However, it is reported in association with bacteria such as Erwinia spp. The symptom of this disease is the development in the rind of light brown, dry, corky spots, which may enlarge and merge to form rather extensive necrotic areas that rarely extend into the flesh. Although there are no external symptoms of rind necrosis, infected fruits appear to have exceptionally tough rinds. Although the mode of transmission is unknown for this disease, it is apparently limited to fruit infection. Watermelon varieties differ in the relative incidence and severity of rind necrosis.
Fruit Blotch
Fruit blotch is caused by the bacterium Acidovorax avenae subsp. citrulli. The fruit blotch bacterium can cause seedling blight, leaf lesions and fruit symptoms.
First symptoms in watermelon seedlings appear as dark water soaking of the lower surface of cotyledons and leaves followed by necrotic lesions, which frequently have chlorotic halos. In young seedlings, lesions can occur in the hypocotyl, resulting in collapse and death of the plant. Leaf lesions are light brown to reddish-brown in color and often spread along the midrib of the leaf. Leaf lesions in the field do not result in defoliation but are important reservoirs of bacteria for fruit infection.
Symptoms on the surface of fruit begin as small, greasy-looking water-soaked areas a few millimeters in diameter. These enlarge rapidly to become dark-green, water-soaked lesions several centimeters in diameter with irregular margins. Within a few days, these lesions may rapidly expand to cover the entire surface of the fruit, leaving only the groundspot symptomless. Initially, the lesions do not extend into the flesh of the melon. With age, the center of the lesions may turn brown and crack, and a fruit rot may develop. White bacterial ooze or effervescent exudate follows fruit decay. Fruit blotch bacteria may be introduced into a field by infested seed, infected transplants, contaminated volunteer watermelons or infected wild cucurbits.
             

Sowing

First possible period of seed drilling (direct sowing) in moderate climate region of Bosnia, Bulgaria, Croatia, Hungary, Romania and Serbia is 20-30th of April. More southern states like Greece, Italy, Macedonia or Turkey sow earlier. Grafted plant can survive on few degrees lower temperatures than regular, but the aim of direct sowing is not the early production. It is risky to do very early sowing. During some seasons (2004) May could be very cold so it’s better to stick to the rule of better safe than sorry. Second sowing raises the cost. Availability of P is reduced in cold soil. Measure land temperature at 10 cm deep in the soil to determine proper sowing time. Do not sow before measuring 10-12º C level inside. There are 9-11 seeds in one gram. Best way to acquire right plant density is to use precise pneumatic drilling machines. Watermelon seed keeps germination for 4-5 years so there is no need to sow more seed than needed. If germination figure on the package states 90% than we use 10% more seed. We should avoid old practice to use more seed (2-2,5 kg) and then do removing of extra plants. This is unnecessary work and plants are fighting for space. There is a strong recent trend among growers to use hybrid seed in direct sowing. Price of this seed is considerably higher than O.P.’s (Crimson Sweet) but benefits of hybrid seed are obvious. For non grafted plants in the fields without irrigation density should be 5.000 –6.000 plants/ha and if there is irrigation system then 4.500 plants are optimal It is not the problem if there are some empty spaces in the field as it will not affect yield. Rows are 2,0-2,5 m apart with 1,0-1,2 plants per running meter (5.000 pl/ha). For grafted plants recommended density is 3.500 – 4.000 plant. For 4.000 plants space between rows is 1,8 m and within rows is 1,2 m. Insure that early watermelons (7-9 kg) have 2-3 m2 per plant, medium sized (9-11 kg) have 3-4 m2 and later large fruited take 4-5 m2 per plant. For seedless watermelon on every 2 rows plant one row of pollinator (regular WM). It is not recommended to do direct sowing of seedless WM as they germinate very slowly at low temperatures (require minimum 26ºC, optimum is 35º) and germination figures are lower than with regular (seeded) watermelons.

If the seed is of smaller diameter and parcel is well cultivated than seed should be placed at 3-4 cm. On lighter soils larger seed should be placed at 4-6 cm deep. After the sowing it is necessary to irrigate and do the regular breaking of surface crust. Irrigation could be done from moving tank after sowing or by using sprinkler or drip irrigation system.

Plant care

There are regular and specific measures of plant care on the field during the vegetation period. Regularly we do cultivation, irrigation, fertilizing (or fert-irrigation) and crop protection. Specific measures are thinning of plants, mulching, putting and removing small tunnels, plant pruning, securing and moving the plants from the wind.
Cultivation
Few cultivation are sufficient. Frequency depends on weed presence and is necessary to keep field clean before plant mass completely covers the surface.  
First in- between row cultivation has to be done at 12-15 cm depth. Very often a hard layer in the soil has been formed by  rain and machines passing through. This hard layer of soil  has to be broken.

Fertilization

If  40-50 tons of manure are inserted than additional 70-80 kg of N, 80-90 kg of P and 80-100 kg of P active matter have to be added. Without manure add 20% more of quantity. Half of N, half of K (K2O) and two thirds of P (P2O5) are put as primary feeding in autumn. Rest goes through 3-4 side dressing during the season. Insert fertilizer  8-10 cm deep in the ground. If mulch foliage has been used and if “classical ”mineral fertilizers is the choice than in autumn add 60% with deep plowing. Suggested NPK formulation is 8:24:16. The rest goes before planting (or sowing) with formulation 15:15:15.  We can’t do side dressing at the mulch so foliar (through the leaves) feeding is necessary if there is no drip irrigation. Dripping tapes are put under the foil next to the plants.

It is best to do soil nutrient analyses in order to make most appropriate fertilizing program. It is our aim to feed the plant not the soil. Also in some areas there is problem   
of high acidity in the soil. Watermelons prefer neutral pH of 6,5 so there could be a need to do calcification with limestone few months before planting.

Constant plant feeding is necessity if we want high yield. Many growers stop with fertilization after the first round (circle) of harvest. High yield means 2-3 harvest circles. Demand for fertilizer is high and for instance at the yield of 50 tons watermelon takes out   100 kg of N, 80-90 kg P, 120 kg. of  K and 20 kg of Mg of active matter from the soil! Microelements such as Fe, Mn,Zn, Cu and especially boron-B (for flower development) are important as well. Manure has all of necessary microelements, activates bacterial nitrification processes and improves soil structure.

If specialized hydro soluble fertilizers are used (Kemira, Haifa, Hydro, Scotts) and especially if feeding is through drip irrigation tapes (Queen Gill, T-tape Ro-drip) please consult representatives for recommendations on formulation and concentration. Every fertilizer company has differently formulated product for designated plant development stage. Plants need a lot of N in initial stages in order to develop sufficient plant mass but also too much nitrogen in fruit development stage postpones harvest and affect quality (bad taste and flesh structure).  For good root and flower development right amount of phosphorus (P) is necessary. For good fruit quality (color, sugar content) potassium (K) is essential.

Nutrient demand in kg/ha/day to be given  through drip irrigation system

Plant stage

N active matter

P2O5

K2O

MgO

Planting-flowering

0,5

2-3

1-2

 

Flowering-fruit setting

1

1-2

2

0,2

Fruit setting-beginning of ripening

2

 

2-3

0,5

Ripening and harvest

0,5

 

1

 

 

Of course that doesn’t mean to add together all those daily needs in few feedings! High EC (concentration) of solution will burn the plants! Keep EC for fruity vegetables generally at 2-3 mmS (mili micro Siemens) or maximum concentration of 0,5%. 

One example of full program with drip irrigation on average soil (carbonated chernozem):
Basic fertilizing in autumn: 450 kg of mineral fertilizer 8:16:24 (N=36 kg, P2O5=72 kg, K2O=108 kg)
or hydro soluble fertilizer (of above mentioned companies) 300 kg 13:11:20 + 2MgO (N=39kg, P2O5=33kg, K2O =60 k, MgO=6 kg).

Further (additional) fertilizing through the drip irrigation system during the season:

Week

Formulation
Ratio

Total
quantity
of fertilizer

N
(85 kg/ha projected)
active matter

P2O5
(55 kg/ha projected)

K2O
(200 kg/ha projected)

MgO
(40 kg/ha projected)

April 20th planting

4th week

NPK 1:4:1

50 kg/ha

5,5

22

5,5

May

1 st week

NPK 1:1:1

50 kg/ha

10

10

10

2 nd

NPK 1:1:1

50 kg/ha

10

10

10

3 rd

KNO3 + Mg(NO3)

50 kg/ha

6

21,5

6

4 th

NPK 2:1:4

50 kg/ha

8

4

16

1

June

1 st week

KNO3 + Mg(NO3)

50 kg/ha

6

21,5

6

2 nd

NPK 2:1:4

50 kg/ha

8

4

16

1

3 rd

KNO3 + Mg(NO3)

80 kg/ha

9,6

34,4

7

4 th

KNO3 + Mg(NO3)

80 kg/ha

9,6

34,4

7

July

1 st week

KNO3 +Mg(No3)

80 kg/ha

9,6

34,4

7

Summ (kg) A.M.

82,3
 given

52
given

203,7 given

35
given

 
Irrigation

Watermelons consist of 90% of water. Most important period for irrigation is during intensive vegetative growth. Second critical period is during fruit setting. Evapo- transpiration (ET) in watermelon crop could be as high as 7,5- 8 l/m2, Quantity of water per irrigation with sprinklers should be up to 40 mm (40 liters per m2). This is general principle but quantity depends on rainfalls and structure of the soil. Clay and humus are keeping the water in the soil. Sandy soils require more frequent irrigation.
From planting till starting of intensive plant growth irrigate every 5-6 days with 10-15 l/m2 or when the land is dry at 15 cm deep.
 From beginning of vegetative growth till flowering irrigate every 5 days if the weather is hot and dry. If the plants are losing tonus in the morning irrigate every 3-4 days.
From flowering till harvest irrigate with 25-30 l/m2 every 4 days except when rainfalls occur. If temperature is above 37º C and air is dry irrigate every 3 days. When the fruit is 10 cm in diameter it is very important to do the irrigation. Fifteen days before harvest onwards do not irrigate.
 That is the program for sprinkling. If irrigation is done by drip tapes than do irrigation more frequently with less water. Drip irrigation saves 40% of water. Main purpose of irrigation is to have enough water at the soil layer 20 and 40 cm deep as where the root system is.

Few tips

It is worth to mention importance of insects for good fruit setting. Without them it is not possible to transfer pollen from anthers to the pistils. Many growers learned about insect necessity the hard way by forgetting to remove small tunnels during flowering. They got misshapen fruits as a result. For optimal fruit setting every flower have to be visited by insects (bees and bumble bees preferably) at least 7 times.  More insects is more seed, larger fruit and less aborted fruits. Some growers put bees next to the field, some treat the plants with   cranberry syrup solution to attract the bees. If bee hives are near the field chemical treatment with insecticides have to be done in the evening when bees are secured and closed inside the boxes.

If agro textiles (Agryl, Kovertan) are used than insect presence it is less of the problem than with closed short plastic tunnels but do few uncovers to trap them.. Agro textiles are strongly recommended for early watermelon (and sweet melon) production.   

To induce early harvest we could prune main plant stem so it could develop more side shoots with more small fruits. For prolonged harvest and bigger fruits opposite direction would be to remove first fruit and let plant form broader vegetative mass.

Harvest, handling and sales

Every grower has it own way to determine when to harvest. Watermelons reach maturity approximately 45 days after blooming. Time to harvest is when the tendril nearest the melon is wilting and the ground spot has turned from white to yellow. Fruits have hollow and dull sound when thumped. Ripened fruits are shiny with nice coloration. Immature WM have turgid tendrils. Completely wilted tendril and pale color can be indicative of over maturity.   Watermelons unlike sweet melons can’t ripe after picking. Sugar content could reach more than 12% level and minimum is 10%. Dry matter (Brix) at 10% level. Harvest WM in the afternoon when they have lower turgor, so there is  less chance of cracking. Do not wash the fruits with strong water flow as it will remove wax layer and reduce shelf life. Also fruits have to be put in the shade to prevent sunburns. Immediate pre cooling after harvest is important for long distance transportation. Watermelons could be stored for two weeks on 15º C. Temperature of 12º C is optimal for longer storage with relative air humidity (RH) of 85%. Flesh loses its color at 10º C. At  8ºC internal structure will start to deteriorate . Do not store watermelons with sweet melons as their ethylene emission would shortage watermelon shelf life considerably.
If we do not have pre contracted production than planting (sowing) in different terms enables wider harvest (sales) frame. There are periods during the sales season when prices rise due to reduced offer (product shortage). Choosing right variety with early, mid-early, mid-late and late maturity is important to make a sequence. If the fruits have to be transported in loose state than thickness of the outer layer (core) is essential. Aim is to withstand pressure while packing 4-5 layers in the truck. Straw layer at the base could help to prevent cracking. In order to prevent damage the best way to transport watermelons is to use calibration machine and then pack them in special carton boxes. West European and Scandinavian market prefers boxed smaller fruits while Balkan states prefer very large fruits in loose state. Good external appearance is important to the customers (rind color, wax layer) as it represents freshness.

As the prices vary profitability of production can be evaluated after 5 straight production seasons. Growers that have stabile yield of 50 tons can find their profitability margin lower than those who reached 20-30 tons.

Very important stability factor and the proper way to insure production is investment in irrigation system. Dry season growers with possibilities to irrigate make larger profits. Many areas are specialized in watermelon production. Reliable quantity and quality always attracts potential wholesale traders. One grower can hardly invest in calibration, handling and proper storage facilities. Negotiating with supermarkets and traders requires time but also dedicated persons with negotiating skills. All of it is easier to organize if growers form co-operatives.

autor

Dipl. Ing. Miroslav Bedov
Development Manager for south-east Europe
E-mail: miroslav.bedov@nickerson-zwaan.com
+381 65 3667050 +381 21 366705