Early season heat waves and periods of drought are becoming the norm, not the exception worldwide. If you don’t prepare for them, you may end up with nothing more than a garden full of dead tomato plants.
For at least a few weeks every summer where I garden in Pennsylvania, it feels like a jungle deep in Central America. Humidity is off the charts and temps roll above 100 degrees.
Heat waves can do incredible damage to even the most heat tolerant shrubs and flowers, but it’s especially damaging to vegetables and fruit. Unfortunately, that damage is not always as easy to detect quickly as it is with frost, which kills many plants immediately.
According to the American Horticultural Society, “Heat damage can first appear in many different parts of the plant: Flower buds may wither, leaves may droop or become more attractive to insects, chlorophyll may disappear so that leaves appear white or brown, or roots may cease growing… When desiccation reaches a high enough level, the enzymes that control growth are deactivated and the plant dies.“
Sounds like plant torture. It’s definitely torture for the gardener who has to witness this slow death of their favorite plants. But how you react when you discover your beloved tomatoes wilting in the scorching sun or tulips gasping for a last breath during a spring heatwave will determine if they make it through the season or give up the ghost.
How sudden, extreme heat waves affect plants
Heat waves affect every species of plant differently, depending on the ancestral origin of the plant – whether its genes first developed in a cold area of the world (cabbage, tulips) or a hot area (maize, cactus). For instance, broccoli reacts quickly to heat at 86 degrees (30 C), but corn tolerates heat up to 100 degrees (38 C). Most food crops can survive sustained high temperatures in only a very narrow range, and few will survive temps exceeding 113 degrees Fahrenheit (45 C).
Plants actually adapt to higher temperatures as the season progresses, and that’s why so much damage is wrought by early-season heat waves, as the plant is in no way prepared for it. Eighty-five degrees Fahrenheit (29 C) in early spring affects a plant much differently than the same temp in summer. Add a baking, intense sun to the mix and plants typically experience sunburn and sunscald and some develop spots or scorching on their leaves.
The timing of the heat wave matters
The type of damage a plant suffers during heat waves depends on its stage of development.
Early in the season when they’re at their most vulnerable, seedlings may quickly die, flower blossoms will drop, fruit will not set, growth will slow or stop, and newly planted seeds may fail to germinate. If heat occurs early enough, pollen won’t develop.
Later in the season, flowers and pods will drop, vegetable and fruit production will stop or possibly speed up, semi-ripe fruit may become discolored and fail to develop further. Additionally, leaves may drop, root development may stop, flowers may become deformed, healthy green color will fade, or the plant may become sterile.
Any of these injuries will negatively affect the number of flowers, foliage, fruits, and vegetables the plant produces throughout the season, as its growth cycle is interrupted. In the case of ornamentals, if the foliage drops or dies but the plant survives, it may not have enough stored energy to produce blooms the following season.
And then come the pests
Heat waves may also affect the balance of insect life in your garden. As wildlife seeks to stay cool, they feed less often. This allows insect populations to suddenly explode. This could be good or bad, depending on which species are present when the heat wave strikes. In most cases though, this is disastrous for plants already stressed from heat. The stressed plant also becomes more vulnerable to viral, bacterial, and fungal infections.
The good news is, if you plan your garden appropriately and learn how to nurse your plants through heat waves, your losses will be minimized and growth will return to normal when the heat breaks.
Read part 2: Designing your garden to withstand heat waves.
Read part 3: Tips on watering plants during a heat wave.