The first hot spell of this year arrived the last week of April, at the usual time. We often see several days of temperatures well into the 90s or even over 100 degrees during the last of April and the first week of May. Well-established landscape plants and trees with large root systems are seldom affected by late spring hot spells (depending on the duration and intensity of the high temperatures), but new transplants will show signs of drought and heat stress.
The arrival of hot days and warm nights in late spring is the signal that the spring planting season has ended. It’s too hot to plant anything during our summers (and too much water will be wasted trying to keep new plantings alive in summer).
Many gardeners have cut back on the planting of ornamental summer-flowering annuals as well as vegetable gardens and reduced or eliminated spring landscaping plant replacements this fourth drought year due to concerns about water supplies and mandated water restrictions. The soil around any new plantings should be covered with a three to four-inch layer of mulch and the plants should be on water-efficient irrigation systems: drip emitters, micro-sprinklers, bubblers, drip tape, etc. Provide shade for young seedlings and transplants from the hot afternoon sun with market umbrellas or shade cloth structures.
It takes about two weeks for the root systems of new plantings to begin to grow after transplanting and about three to four weeks for immature root systems to begin to become established. Until root systems are established, they aren’t able to draw up enough water to keep the plant from becoming stressed. Wilting, leaf scorching, and even death (terminal wilting) are caused by insufficient water as well as heat stress. New plantings will require extra watering for the first three or four weeks; during late spring hot spells, new transplants may need watering every other day; seedlings will need watering every day.
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Late spring hot spells also cause young plants’ growth to become stunted and they also cause flower drop. New plantings that are stunted from heat stress will not recover if temperatures remain higher than normal — as is often the case during drought years. Monitor the growth of heat-stressed transplants and, in order to conserve water, pull them out if they’re not growing.
During warmer-than-normal drought years, fruit and flower set can be hit and miss during the summer growing season. Tomatoes, cucumbers and bean seeds that were put in the garden at the end of March when temperatures were already into the 80s had flowers on the vines last week. Bean, tomato and cucumber flowers will drop when temperatures are regularly above 90 degrees. Fruiting hormones won’t help flower or fruit set when temperatures are high. Eggplants and peppers tolerate heat better and will set flowers and fruit when temperatures are above 90 degrees.