Successful weed management is vital to the production of quality watermelons. Weeds compete with the crop for light, space, nutrients and, particularly, water. Weed growth promotes disease problems and can harbour deleterious insects and diseases. Watermelons, as with most crops, require early season weed control to ensure a quality crop. In addition, the spreading nature of this crop makes weed control difficult once the vines begin to form.

Weed identification, especially seedling weeds, is also important. Seedling weeds are generally easier to control and in many cases control can occur only at the seedling stage.

Another important factor is the growth of the crop. Generally, an aggressive, healthy crop will out-compete and exclude many weeds. Proper fertilisation as well as disease, nematode and insect management will promote crop growth and aid in weed suppression.

Several researchers have indicated that weeds play an important role in the spread and epidemiology of plant viruses. These weeds can be a source of virus in their seeds, the overwintering sources of the virus.

In studies carried out to survey for the presence of naturally occurring cucumber mosaic virus (CMV) in weeds in Malaysia, the virus was detected in many weed species e.g. Ageratum conyzoides and Crotalaria spp., In another study squash mosaic virus (SqMV) was detected in A. conyzoides, Cleome rutidosperma, Croton hirtus, Euphorbia hirta , Euphorbia prunifolia, Eupatorium odoratum, Hyptis brevipes and Mikania micrantha whilst zucchini yellow mosaic virus (ZYMV) was detected in A. conyzoides and Wedelia trilobata . In one sample of A. conyzoides , SqMV and ZYMV were detected simultaneously while SqMV and CMV were found in samples of E. hirta and E. odoratum.

Several methods of weed control exist for watermelons. Selecting the method best suited for an individual grower will depend on several factors: weed species , stage of crop and weed development, and labour cost and its availability.

Hand weeding provides very effective weed control and is safe to the crop. Weeding should be performed when the crop and weeds are small to reduce crop damage and to allow hoeing. Removal of large weeds with extensive root systems may damage crop roots or vines. Hand weeding, however, is costly in terms of labour.

Mechanical cultivation provides very effective weed control but is limited to small weeds that can be easily uprooted or covered. More importantly, mechanical cultivation should not be performed once the plants have begun to vine ("run"). These vines are very tender and are easily damaged by tractor wheels or cultivators. Mechanical control must be supplemented with chemical or hand weeding to remove weeds in the rows or after the plants produce vines.

Weed control using the stale seedbed technique involves chemical weed control of emerged weeds before crop emergence. A non-selective contact material is used e.g. alachlor (sprayed at a rate of 4 litres per hectare). The stale seedbed method often is coupled with a pre-plant-incorporated herbicide treatment. If the crop is transplanted, this method may be used to kill emerged weeds before transplanting. On direct-seeded plantings, apply the herbicide to those weeds that have emerged after planting but before the crop has emerged.

Plastic mulch with drip irrigation is a very effective method of weed control. Plastic mulch also reduces the loss of nutrients due to leaching especially in well drained and sandy soils.

Leaching is a process where nutrient solution moves downwards with ground water during heavy rain or irrigation. Nutrients that are moved down to below the root zone will not be available to the plants and will pollute ground water.

Black or non-light-transmitting plastic is preferred because it eliminates light required for weed germination and growth. In Malaysia, `silver-coated' black plastic is commonly used as a plastic mulch. Plastic that covers the plant beds should fit tightly and seal the edges to prevent wind disturbance. When fixing the plastic mulch, make sure that the silver coloured side is facing the sky.

The plastic mulch may be fixed mechanically or manually. The plastic mulch can be mechanically fixed with help of tractor mounted plastic mulch fixer.

Once the bed is covered, a small hole is made in the plastic and the transplant or seeds inserted. The smallest hole possible is advantageous to eliminating weed emergence. Those areas between the beds should be treated only with an herbicide registered for the crop because the crop roots may extend into the row middles and contact the treated soil.

Pruning of the watermelon vine has been shown to produce uniform size fruits. This has the advantage of shortening the harvesting period due to the uniform ripening of fruits. The main vine is cut at the sixth or seventh node when the plants are 15 – 20 days old. 

Water is a critical component in the production of watermelons. A ripe watermelon consists of more than 90 percent water. Thus, an adequate water supply is critical to optimising yield and quality of this crop.

Water deficits during the establishment of watermelons delay maturity and may cause gaps in production. Water stress in the early vegetative stage results in reduced leaf area and reduced yield. The most serious yield reductions result from water stress during flowering and fruit development.

Several types of irrigation systems may be used successfully on watermelons. Ultimately, the decision about which type to choose will be based on one or more of the following factors: