The watermelon is an annual herbaceous trailing plant which belongs to the cucurbitaceous family and goes by the scientific name of Citrullus lanatus.
As is common to intensive crops, the market life span of this fruit has been extended due to the fact that it is a forced crop which is protected by being grown in greenhouses. This means it is no longer a fruit grown exclusively during the summer only.
The process of planting of watermelons in greenhouses is carried out with seeds which have previously been germinated in seedbeds. This takes place from December until the month of April, which means they are sown earlier than they would be if sown directly.
Germination begins as soon as the cotyledons appear. Once the seedlings have grown to a height of about 8cm the end bud begins to appear, becoming longer, until the beginnings of new leaves start to show. At 10 or 12days after germination the cotyledons begin to wither and the end bud begins to sprout.
Crops which are grown under plastic are subject to the kind of temperatures and humidity that favour the spread of diseases. In order to overcome this, pumpkin pattern grafting is carried out due to its morphological and physiological affinity to the majority of watermelon varieties grafted. This creates a resistance towards Fusarium wilt and other fungus found in the soil.
The time lapse between sowing and transplanting varies between 30 to 40 days from the point of initial sowing in seedbeds. Seedlings should be transplanted into the soil with the root balls in tact; as this plant cannot stand to be transplanted with bare roots.
Holes are made with a hoe or a dibber. Once the root ball has been set in place it is covered with soil and watered, which helps to stabilize the plants and facilitates the root taking process.
One month after nascence the watermelon will begin to develop trailing stems and show the beginnings of stalks that branch out. A second watering should be carried out 10 or 12 days after this takes place.
Once watermelon plants are showing several leaves, some superficial work needs to be done in order to get rid of weeds and so that the technique known as ridging may be carried out.
There are two methods of pruning which are appropriate for watermelons cultivated in greenhouses.
The first requires action as soon as the plant begins to grow, which means that as soon as 4 or 6 true leaves appear, they should be de-budded above the leaf, causing the axial bud of this leaf to be pinched. The same process is repeated with the secondary branches. In this way the adult plant will have two stalks that grow out from the main stem and four secondary shoots.
With the second method only the two main branches are left to begin with. When these branches have 5 to 6 leaves, de-budding is carried out above the third leaf so that four secondary branches are allowed to form.
In some regions it is quite common to cover the soil with compost, plastic strips or sand etc. In this way the temperature of the soil can be maintained at a more appropriate level for this type of crop. In addition to this, soil humidity is maintained, weeds are kept under control and work on the soil is reduced.
During the flowering stage, the flowering buds will give place to solitary male or female peduncle axial flowers. The first flowers to appear are the male flowers, followed by the female flowers. The latter appear on all the branches, the female flowers normally grow on the main branches and secondary shoots.
Due to the absence of natural pollination, hormone substances are applied in greenhouses in order to cause the fruit to grow.
The ovary develops and becomes a fruit due to pollination and fertilization. This is a strictly hormonally controlled phenomenon which is influenced by the conditions within the greenhouse, together with the light intensity available. If the level of hormones used is not that which is required, then phytoregulators may be used to correct the deficiency.
The earliest variety will begin to develop fruit a month and a half after nascence and at the two month stage will show stalks of about 1 metre in length. From this moment onwards, the plant will require a period of prolonged warmth in order to assist the fruit during the ripening stage, which ends 80 to 100 days after nascence.
The date for harvesting the watermelon is dependent upon the variety being grown, the area, the time of sowing and the crop system used. A time period of two to three months will elapse from sowing or planting to the commencement of harvesting. This time period will be prolonged by 7 to 10 days with respect to grafted plants.
The best times to pick the crop are at dusk or early in the morning. Watermelons are cut leaving 2 to 3cm on the peduncle. It is important to make a clean cut in order to avoid damaging the stalk.