The Heat is On!

By Steve Reiners, Professor and Chair, Horticulture Section, Cornell University, Cornell AgriTech. This is part of a series of columns that he wrote about vegetable gardening during the pandemic that ran Spring-Summer 2020. Read more articles from 2020 | Read later articles in this series.

July 10, 2020

After a cool spring, the last few weeks have been hot and dry in upstate New York.  Although we can water to help with the drought, there’s not much we can do to protect our plants from the heat.  And unfortunately, it can lead to lots of problems in our gardens.

Cool-season crops like peas and spinach just can’t take the heat.  At Cornell AgriTech, my research includes an acre of peas with dozens of varieties.  High temperatures are causing the peas to ripen quickly, from sweet and tender to hard and starchy in 24 hours.  Spinach responds to heat by flowering and going to seed.  Lettuce bolts too, but may take the heat a little better.

Heat impacts squash, zucchini and cucumbers both directly and indirectly.  High temperatures lead to more male flowers at the expense of females.  And it’s the female flowers we need to get fruit.  Plus, hot days keeps bees from foraging and pollinating.  So even if you have female flowers, you may notice them fall off as bees failed to do their job.  

Tomatoes are not immune to the heat either.  Although we think of them as heat loving plants, they prefer more seasonable summer temperatures.  Hot days over 90F and warm nights over 80F results in pollen sterility.   Similar to squash, without viable pollen, you may notice flowers and pea-sized fruit fall off the plants.

The condition is temporary, and fruiting will return to normal when it cools down.  But that might explain the lack of fruit you might see about six weeks from now.  That’s just about when the fruit that should be setting now would be ready for harvest.  Peppers too have a similar problem with fruit and flowers falling off.

Snap bean flowers may also abort in the heat.  That’s not a big problem for gardeners as production picks up again when the weather cools down.  It is a huge problem for commercial growers, however, since they don’t have the luxury of picking a few beans every day.   They get one chance to harvest their field with mechanical harvesters, so they need everything to ripen at one time.

The heat causes quality problems too.  Cucumbers become more bitter due to a high level of cucurbitacin, a compound that occurs naturally in the fruit but usually at low levels.  Stress of any kind tends to increase its level.  The compound tends to accumulate in the stem end of the fruit and the skin.  Just cut off the ends and peel to make a more palatable fruit.

Beets and carrots tend to lose color (and phytonutrients) as well as stored sugars in the heat.  Better to wait until the weather cools down to harvest.  Wait until autumn to harvest for the best root crops.

Minimizing heat problems

Is there anything you can do to minimize heat problems?  First thing to do is keep up with watering.  Dry soils make problems worse.  To keep plants from wilting and drying up completely, water needs to move continuously from the soil into the roots and up through the leaves.  The water is needed for photosynthesis which of course powers the plant.  But it also cools the leaf as it evaporates.  Just like people, evaporation keeps plants cooler.

If it’s dry,  the small pores on the leaves called stomata close. That stops water from evaporating from the leaves.  This not only shuts down photosynthesis but causes leaves to heat up.

Midday sprinkling helps cool plants by increasing evaporation from leaf surfaces. Water lightly as your goal is to just wet the leaves and not provide a week’s worth of irrigation.  Don’t overuse this method as it can increase disease problems due to the higher humidity.  But it can help on the hottest days.

You can also try throwing some shade cloth over plants on hot days.  Depending on the type of cloth, it can reduce the sunlight and the temperature a bit.  This technique is probably best for cops like lettuce and spinach.

Now is a good time to add a layer of organic mulch to your soil.  Wood chips, straw, shredded paper and grass clippings will reduce evaporation of water from the soil and keep it cooler.

This week in the garden

Keep up with harvests. Most fruits and vegetables are best picked when on the small side.  As zucchini, cucumbers, and beans enlarge, so do the seeds.  They also get more fibrous and starchier.  Your plants will be more productive too and produce  more fruit if you harvest at the right time. Still time to plant snap beans, cucumbers and summer squash. And keep up with the watering!