Vigorous plants such as courgettes, marrows, pumpkins and squashes can make huge amounts of leafy growth but this doesn’t always equate to good flowering or fruiting. There may be a number of reasons for this:
Lack of female flowers
Note – female flowers can be identified by the swelling of the immature fruit at the base of the flower.
Immature plants tend to produce only male flowers early in the growing season. As the season progresses the production of male and female flowers is more balanced.
Low temperatures, lack of sunshine and adverse growing conditions can also be accountable for lack of female flowers.
- Do not plant out too early; wait until the risk of frost has passed
- Protect plants during cold nights in early summer with fleece or cloches
- Water regularly to prevent drought stress
- Apply a balanced fertilizer to promote stronger growth. Feed vigorous plants with a high-potassium feed such as a tomato feed
- Avoid planting in a shady position and overcrowding the plants
Lack of male flowers
High temperatures during periods of hot weather or in a poorly-ventilated greenhouse can result in a lack of male flowers. This problem is most common in late summer and autumn.
- Avoid planting out transplants too late in the season (after mid-summer)
- Ensure good ventilation if plants are grown in the greenhouse or under cloches
Plants will naturally produce fewer male flowers as the autumn approaches.
Fruit set and development problems
Cucurbits, with the exception of glasshouse ‘all female’ cucumber cultivars, produce separate male and female flowers that require pollination to set fruit (note: older glasshouse cucumber cultivars produce male and female flowers but if pollination occurs the resulting fruits are bitter so this must be prevented by removing the male flowers). Pollination is the transfer of pollen from the male flower to the female flower and is usually carried out by insects.
Environmental factors and unsuitable growing conditions can be the reason why plants fail to set despite the plant producing both male and female flowers. The young fruitlets are aborted and will drop off. Sometimes, the fruit will begin to develop, swelling mostly at the stalk end. The flower end of the fruit will often shrivel up and start rotting. The reason may be:
- The plants may be too young and small to sustain fruits.
- Lack of pollination due to low activity of pollinating insects, mainly honeybees, during periods of poor and cold weather. Hand pollination will ensure transfer of the pollen:
1. First identify a male flower. Male flowers differ from female flowers by not having a fruitlet at the base of flower
2. Pick off a male flower from the plant, then carefully remove the flower petals to expose the pollen-bearing anthers
3. Press the male flower into the centre of the female flowers. One male flower will pollinate several female flowers
- When the crop is grown under cover such as a greenhouse or cloche where the insect access is limited. Leave doors and vents open on hot days to encourage insects, but hand pollination may be necessary.
- Plants that are stressed and weak through unsuitable growing conditions. Feed and water regularly especially container-grown plants.
- Failure to pick the mature fruit regularly. This is the plant’s way of regulating the number of developing fruits that it is able to support. Ensure regular harvests of mature fruit or, in the case of courgettes and cucumbers, immature fruits.
As the autumn approaches growth rate and fruit set will be naturally lower.
Bitter tasting fruit
The bitter taste of some fruit is caused by an over-production of plant defence chemicals called ‘cucurbitacins’. This is mainly a problem in courgettes and summer squash and is caused primarily by a mutation within the plant. The problem is more likely when plants are grown from saved seeds, where inadvertent cross-pollination may have occurred.
Affected fruit should not be eaten as it causes stomach upsets and affected plants should be removed.