A heatwave will likely cause many home vegetable gardens to experience a drop in production. Problems associated with extended periods of temperatures in the 90s include blossom drop, blossom end rot, and poor pollination, all of which result in little to no produce. Depending on which problem your garden is experiencing, there may be something you can do to help your vegetables stay productive during the heat.
Temperatures in the nineties cause many vegetables to drop their blossoms before fruits form. Squash, zucchini, tomatoes, and beans are especially prone to blossom drop.
To minimize blossom drop during a heatwave, keep your garden well watered by applying water twice a week if it does not rain. The best way to water your vegetable garden is with soaker hoses or a drip irrigation system that delivers water at the soil level. This reduces water loss to evaporation and keeps plant leaves dry, which minimizes foliar diseases such as leaf spot and early blight. Mulch can be used in vegetable gardens to conserve moisture and keep soils cool.
You should also avoid liquid fertilizers that contain high levels of nitrogen (those with a nitrogen content over 10% – this does not include compost tea or most fish emulsion/seaweed products). Nitrogen causes plants to produce lush, green leafy growth instead of flowers and fruits. Using organic fertilizer or time-release fertilizers such as Osmocote will reduce the risk of over-fertilizing as these products release their nutrients slowly over time.
Many summer vegetables must be pollinated by bees to set fruit. During the pollination, process insects transfer pollen from the male part of the flower to the female part. For fertilization to be successful the pollen has to be transferred at the right time and the pollen must be viable.
During hot weather, pollen does not remain viable very long – it literally spoils in the heat. As a result, you will likely experience a drop in vegetable production for several days during and after a heatwave even if you see lots of bees visiting your garden. You may also see more misshapen fruit as a result of poor pollination.
Little can be done to prevent weather-related pollination issues. Gardeners can help protect pollinators by minimizing pesticide use and selecting products less toxic to bees. If you need to apply a pesticide in your garden, do so only in the late evening when pollinators are less active. Also, avoid using dust insecticides, such as Sevin dust, because these products stick to bees’ bodies are transported back to the hive, where they are spread among the colony.
Blossom End Rot
Another problem that is common in hot weather is blossom end rot (BER). This disorder occurs when there is not enough calcium available for developing fruit, resulting in the lower end of the fruit turning tan or black and failing to develop. BER is most often seen in tomatoes, but can also occur in eggplant, peppers, watermelons, and squash. The most common causes are uneven watering, over-fertilization, extreme temperatures, and low soil pH (acidic soil).
If your crops continuously develop BER, submit soil samples to the NC Department of Agriculture’s soil testing lab to see if your soil pH is too low. If this is the case, adding lime to the soil will help prevent BER in future crops but will not fix the problem this year. One of the most important things you can do to reduce BER this season is to keep the soil in your vegetable garden evenly moist. Do not allow crops to dry out during dry spells and do not over apply fertilizers.
One last thing you can do to keep vegetables productive all season is to harvest frequently. Most summer vegetables should be harvested every couple of days to maintain productivity. Leaving fruits on too long reduces future fruit set. Be sure to pull large fruits off vegetable plants because they drain the energy necessary for new flower production.
Charlotte Glen is the Agriculture Agent – Horticulture at the NC Cooperative Extension, Chatham County Center in Pittsboro.