Watermelon and fruiting vegetables: lots of flowers, little fruit

Vines grow well and flower, but few fruit form, or small fruit turn yellow and drop. On watermelon, cucumber, melon and pumpkin the cause is very often pollination.

These crops carry separate male and female flowers. Male flowers usually appear first and keep coming through the season; female flowers carry the small fruit at their base. Each cucurbit flower is open for a single day, so if a female flower is not pollinated on that day, it is lost.^[1]

Female watermelon flowers open early in the morning, are most receptive before about 10 am, and close in the afternoon. A watermelon flower needs many pollen grains to set a well-formed fruit — at least eight honey bee visits, according to extension guidance.^[2]

• Too few bees. Although bad temperatures can reduce pollen quality, the more common problem is simply not enough bee activity in the field.^[1]
• Spraying when bees are working. Insecticide sprays during peak bloom or when pollinators are active kill or drive away the bees the crop depends on.^{[1, 4]}
• Too much nitrogen. Excess nitrogen pushes vine growth and delays or limits fruit set in cucurbit vine crops such as pumpkin.^{[2, 3]}
• Heat. High heat reduces pollen viability and germination, so pollination fails even when bees visit.^[2]
• Water stress. Inadequate irrigation reduces fruit set and increases fruit abortion, and drought can raise the ratio of male to female flowers, leaving fewer fruit per plant.^[3]

• Bring in or protect pollinators. Where bees are scarce, growers place honey bee colonies near the field at about the time the first female flowers appear.^[1]
• Time sprays around the bees. If a spray is needed, avoid peak bloom and the morning hours when pollinators are most active; late evening, after bees have finished foraging, reduces the risk to them.^{[1, 4]}
• Go easy on nitrogen. Give enough nitrogen for healthy vines, not so much that the crop stays leafy and delays fruiting. A soil test shows what the soil already supplies.^{[2, 3]}
• Irrigate steadily through flowering and fruit set. Avoid letting the crop go short of water while it is flowering and setting fruit.^[3]
• Hand-pollinate as a back-up. Hand pollination is possible on small plots, although relying on insects is much less labour-intensive. Do it in the morning while female flowers are receptive.^{[2, 4]}

NEXGROW is a bio-organic, phosphate-solubilising fertiliser. For watermelon, the product booklet describes it as supplying nutrients that support photosynthesis, vine growth and branching, and improving soil physical properties by lowering soil temperature and bulk density and increasing porosity. The booklet also has a program for fruiting vegetables.

It is mixed into the soil before planting and does not replace pollinators. Rates below are taken directly from the booklet; see the full crop programs for fruiting vegetables.

Test the soil before planting, so that nitrogen is set to what the crop needs and not overdone. Excess nitrogen is one of the known causes of delayed fruit set in vine crops, and a test tells you how much the soil already supplies.^{[2, 3]}