Growing watermelon –if done rationally and on a scalable basis- can be a good source of income. In a few words, most commercial watermelon growers start the crop from seeds (hybrids) in an indoor protected environment. As they wait for the young seedlings to grow and be ready for transplanting, they prepare the field. They till the land, they make the beds or furrows and they place a black plastic film through the rows. The black plastic film not only helps the soil become warmer but also controls weeds. They also design and place the drip irrigation system. When they are ready for transplanting, they make small holes in the plastic film, where they dig small holes and plant the seedlings. Fertilization, Drip Irrigation and Weed Management is applied in most cases. Thinning is also applied. Commercial watermelon growers remove the malformed or underdeveloped watermelons in order to encourage the plant to devote its resources in fewer but bigger and tastier fruits. Most commercial watermelon varieties can be harvested 78-90 days after transplanting. Harvesting can only be made through hand scissors or knives. After harvesting, watermelon growers plow and destroy the remaining of the crop. They may also rotate the crop, in order to control diseases or prevent soil from depleting.
The restrictive factor when growing watermelon is always the climate. Watermelon plant comes from Africa. It is a plant extremely sensitive to low temperatures and frost. It requires on average temperatures from 18 to 35°C (65 to 95 °F), while soil temperature should not fall below 18°C (65°F).
First of all, it is crucial to decide the growing method as well as the varieties of watermelon that thrive in our area. There are 3 methods to grow watermelons: Growing from seed, growing from non-grafted seedlings and growing from grafted seedlings.
Growing Watermelons From Seed
Watermelons are long-period crops. For growing outdoors, they need on average 100 to 120 days from seeding to harvesting. However, if you are planning to grow watermelon from seed, there are some facts you need to know. First, watermelon seeds require at least 18 °C (65 °F) soil temperature in order to germinate. Second, it is important for the seed to have optimum moisture levels in order to sprout. Over irrigation can be harmful. Some producers water thoroughly the soil a day before sowing and do not irrigate again, until it sprouts. However, this is not a good technique if the soil is too sandy and has difficulties in preserving enough available water.
Watermelon seeds germinate easily in 6-10 days depending on the weather and soil conditions.
In areas with a danger of frost, growers prefer to sow the seeds in seed beds under controlled conditions and then transplanting them into their final positions. They most commonly use turf as substrate for optimum aeration.
Growing Watermelons from Non-Grafted Seedlings
Another commonly used method is growing watermelons from non-grafted plants. If we follow this method, it is crucial to choose carefully the variety of watermelon we are going to plant. If -for example- the fields in our area have problems with diseases, pests, lower or higher pH or salinity levels, then not all varieties can thrive. Some varieties are tolerant to some of those factors, while others are not. The most commonly used varieties are: Charleston Gray, Crimson Sweet, Jubilee, Allsweet, Royal Sweet, Sangria, triploid seedless and Black Diamond types.
Growing Watermelons from Grafted Seedlings
Nowadays, most growers prefer to use grafted watermelon seedlings. Grafting is a commonly used technique by which we join together parts from two different plants, so that they will grow as a single plant. The upper part of the first plant is called scion and grows on the root system of the second plant, which is called rootstock. Eventually, we have a plant that combines all the advantages of its different components. Some producers prefer to grow from seed both the rootstock plant and the scion. Then, they perform the grafting by themselves, while others buy certified grafted seedlings from legitimate sellers. The most commonly used seedlings nowadays are watermelon scions grafted on squash rootstocks.
Soil Requirements and Preparation for Watermelon Cultivation
Watermelons thrive best in rich, slightly sandy soils with pH levels from 5,8 to 6,6. They do not like soggy soils. Heavy clay soils with poor drainage and aeration should be avoided. Watermelon farming requires extensive soil preparation before planting, in order to be profitable and lead to high yields.
The basic soil preparation starts about 5 months before transplanting watermelon seedlings. Farmers plow well at that time. Plowing improves soil aeration and drainage. At the same time, plowing removes rocks and other undesirable materials from the soil. Tillage comes right after plowing. Tillage tractors leave the soil free from weeds which can be harmful for the crop.
One week before planting, many farmers apply a pre-planting fertilizer such as manure or synthetic commercial fertilizer, always after consulting a local licensed agronomist. Since watermelon plants need a lot of space to grow, farmers plant them at predefined distances. Consequently, there is no reason to apply the pre-planting fertilizer to the entire field. A good technique is to mark the areas you are going to plant and then apply the fertilizer towards the lines. The next day is probably the right time to install the drip irrigation pipes. Following the installation, some farmers can apply soil disinfection substances through the irrigation system, in case soil analysis has revealed soil infection problems (ask a licensed agronomist in your area).
The next and most important step (especially in countries with non optimum soil temperature during the planting period) is the linear polyethylene coating. Many producers cover the rows with black or green Infrared- Transmitting (IRT) or black plastic film. They use this technique, in order to maintain the root zone temperature at optimum levels ( >18°C or 65°F) and prevent weeds from growing.
Watermelon Planting and Plant spacing
In many cases, the most suitable period to plant watermelons outdoors, is during the second half of spring. At that time, the danger of frost has passed in most cases. Farmers generally prefer plants aged from 3 to 6 weeks. At this point they have developed maximum 3 veins (ideally 1-2).
After all the preparation steps that started 5 months before planting (plowing, basic fertilization, tillage, installation of the irrigation system and plastic film covering), we can proceed with transplanting. Growers label the exact points on the polyethylene plastic where they will plant the young plants. They then dig holes on the plastic and plant the seedlings. It is important to plant the seedlings at the same depth as they were at the nursery.
As far as the planting distances are concerned, a commonly used pattern for varieties that produce fruits up to 14 kg, is 1m (3,28 feet) distance between plants on the row and 3.5 m (11,48 feet) distance between rows. This pattern will give us 2000-2500 plants per hectare. (1 hectare = 2,47 acres = 10.000 square meters). The distances and the number of plants depend on the watermelon variety, environmental conditions and of course the desired watermelon size that is always dictated by the market. For example, if we plant more seedlings per hectare, we will harvest fruits of smaller sizes. A different pattern for smaller fruit varieties is 1,5 m (5 feet) between rows and 0,6m (1,9 feet) between plants in the row. Following this pattern, we will approximately plant 11.000 plants per hectare. (1 hectare = 2,47 acres = 10.000 square meters).
Watermelons Low Coverage
Due to the fact that in non tropical countries, even in spring, there is always danger for frost or heavy rain, most producers protect young plants with low tunnels coverage. Right after planting, they create tunnels of 50 cm (1.6 feet ) height, using plastic or iron support struts and white plastic covers. In a few words, they create tiny greenhouses so as to maintain the desired microclima and protect the young seedlings from harmful factors.
Approximately 45 days later (depending on the weather conditions), they start to gradually rip the plastic day by day, until they fully uncover the plants. A few days later, they completely remove it from the field. This gradual and incremental tearing of tunnel is very important. Otherwise, the sudden removal of the plastic will stress the plants.
Watermelons Pruning – A controversial method
Some watermelon producers prefer to prune their watermelons, while others claim that pruning delays the development and fruit setting of the plant. Those who prune their plants, remove most of the peripheral veins of the plant early, during the first stages of development, when it only has 3-4 veins. With this method, they force the plant to develop further through the main vein. They keep removing excess foliage that prevents proper aeration, during the entire growing period. Thus, they protect the plant from humidity favored infections like Powdery Mildew.
Watermelon Water Requirements and Irrigation Systems
According to FAO, the total watermelon water requirements during the entire growing period range from 400 to 600mm.
Of course, the water requirements can be totally different under different weather and soil conditions. For example, heavy clay soils normally need less irrigation sessions than a sandy soil. Additionally, high atmosphere humidity or rainy days may not require irrigation sessions at all. On the other hand, a dry day with very high temperature may require one irrigation session per day.
Many producers in Mediterranean countries like Greece, prefer to irrigate their watermelons 20 minutes per day, during their first stages. During the fruit setting stages, and as the temperature has increased enough (>35 oC), they increase irrigation sessions, due to the extended needs of the plant at these stages. Finally, they reduce irrigation dramatically, and almost stop irrigation during the last stages of maturity. Excess water at these stages will cause the fruit to crack. In some states of the USA, commercial watermelon producers provide on average 25mm of water per week. Many producers prefer to irrigate their watermelons early in the morning during the first stages and late in the evening as the temperature increases.
Generally, watermelons have high water requirements, but watering the foliage has been linked with diseases outbreaks. Excess humidity in general may favor the development of pathogens such as Powdery Mildew. On the other hand, water-stressed plants are more susceptible to diseases.
The most commonly used irrigation system is drip irrigation.
Watermelon fruit setting relies on the activity of bees and other beneficial insects who distribute pollen. Especially when we grow seedless varieties, placing 1 or 2 strong and healthy hives per 1 hectare is necessary. Manual Pollination can also be a choice if we grow watermelons inside greenhouses or if the natural bee population in our area is not sufficient to pollinate our plants.
Watermelon Fertilizer Requirements
First of all, you have to take into consideration the soil condition of your field through semiannual or annual soil testing, before applying any fertilization or tillage method. No two fields are the same, nor can anyone advise you on fertilization methods without taking into account your soil’s test data, tissue analysis and crop history of your field.
However, we will list the most common watermelon fertilization schemes, used by a considerable number of farmers.
The most commonly used fertilization method is “fertigation”. Producers inject water soluble fertilizers in the drip irrigation system. In this way, they can provide the nutrients gradually and give the plant the proper time to absorb them.
Nowadays farmers make from 0 to 10 fertilizers applications throughout the 3 months growing period (from planting to harvest). Many farmers apply a pre-planting fertilization such as manure towards the rows one week before planting and start the fertigation 2 days after planting. At this point, they apply a Nitrogen- Phosphorus- Potassium 12-48-8 fertilizer, enriched with trace elements (micronutrients). High Phosphorus levels at the first stages will help plants develop a strong root system. Additionally, in many cases micronutrients make it easier for plants to overcome any stress conditions caused by transplanting.
The next 3 applications (1 per week) alternate between 15-30-15 and 12-48-8.
For the next 4 weeks, they apply interchangeably 20-20-20 and Ca(NO₃)₂ keeping a time window of 3 to 4 days between every application.
For the next 2 weeks, they do not apply any fertilizer. At week 11, they apply 20-20-20 until the fruit reaches ⅔ of its final weight. From this point onwards, they start providing watermelons with KNO3 . At the final maturity stage, they change to Κ₂SO4 . At these stages, plants normally have greater needs for Potassium in order to create big, well shaped fruits with high sugar levels.
However, these are just common patterns that should not be followed without making your own research. Every field is different and has different needs. Checking the soil nutrients and pH is vital before applying any fertilization method. You can consult your local licensed agronomist.
Watermelon Pests and Diseases
The first precaution against pests and diseases is crop rotation. The second is to purchase only certified and disease free seeds and seedlings.
Thrips palmi are slender insects that attack watermelons by sucking the sap of leaves. Sunny and hot weather favor the infestation. Thrips management begins with proper precautionary measures. These include, weed control and crop rotation.
A good technique is to constantly monitor their population. If the number is over the tolerable limits, then you can consider to intervene, always after the advice of a local licensed agronomist. There are biological as well as chemical solutions on the market, which of course should always be used under GAP standards and the supervision of a local licensed agronomist
Aphids suck the sap and cause the plant to weaken. Leaves start to curl and shrink. Furthermore, aphids transmit several virus diseases.
This mite primarily appeared in European countries. However, nowadays it is an American problem as well. It damages leaves, stem and fruit. It causes chlorotic spots on leaves. The mites also cause discolouration on fruits, lowering their quality.
Anthracnose is a disease that causes serious damages mostly on leaves and veins. It is caused by the fungus Colletotrichum lagenarium. Cool and wet weather favors the fungus spores. Dry and hot weather conditions stop the disease cycle, which will continue again when weather conditions will be optimal. Symptoms appear primarily on the older leaves causing brown necrotic spots. We may also observe these infestation damages on stems, flowers and fruits.
Anthracnose control begins with proper precautionary measures. These include: weed control and proper distances between plants, along with proper pruning for optimal aeration. The proper nutrient and water levels of the plants can also boost their immunity. Chemical treatment is used only if the problem is severe and always under supervision from a licensed agronomist.
Downy mildew is caused by microorganisms of the Peronospora or Plasmopara genus. Symptoms most commonly appear on leaves after rain or during days with high humidity (often during spring). When our plants are infected by downy mildew, we will probably spot yellow or gray spots with mildews underneath. Downy Mildew control always begins with proper precautionary measures. These include: weed control, proper distances between plants along with proper pruning for optimal aeration. Chemical treatment is used only if the problem is severe and always under the supervision of a licensed agronomist.
Powdery Mildew is caused by many different species of fungi. However Erysiphales and Podosphaera xanthii appear to be the most common ones. We can actually see a white powdery mildew on the leaves. As the powdery mildew moves through the vessels, leaves tend to become brown and eventually die. Powdery Mildew control includes the same steps as for Downy mildew. We must always disinfect our tools after we have handled an infected plant, in order to prevent the infection from spreading to healthy plants.
Watermelon Harvest Yield and Storage
Most watermelon varieties reach their full maturity and are ready to be harvested 78 to 90 days after transplanting. When they are ready for harvesting, in most cases we notice a yellow spot on their skin on the surface that is in-touch with the soil. Furthermore, we can observe a dry tendril on the part at which the stem is linked to the vain.
Due to the differientations in pollination time, not all watermelons mature at the same time. Thus, we may have to harvest the same field more than one times.
Watermelons can be harvested only by hand. We must be cautious to cut and not pull the watermelon, otherwise the fruits may crack open, and in this case they cannot be marketed.
A good yield, after some years of experience is 50 to 80 tons per hectare. In commercial watermelon farms, we may expect to harvest 1,5 to 2 full size watermelons per plant.
Watermelons are then transferred to cool but not freezing storage areas with a temperature of 10 °C ( 50 °F).
This article is also available in: Ελληνικα