Why Excessive Heat Can Spell Trouble for Your Tomatoes
I have often heard from people how a particularly hot summer spells trouble for their tomatoes. I think back to 2016 - a particularly terrible year for tomatoes. Even though I was able to provide plenty of tomatoes at our growers’ market stall, my yield was only about 30% of what I had projected at the outset of the year! Not to mention that several of the varieties we planted simply never made it to the table. Tomato growing is a notoriously fickle endeavor: When years are bad they can be really bad; when years are good you can be buried in tomatoes. Furthermore, we often don’t know what causes either result. This post explores early and sustained heat as a key factor that can make growing tomatoes particularly difficult.
I Thought Tomatoes Were Heat-Loving Plants?
It is true that tomatoes grow best in temperatures above 60˚F and will immediately succumb to frost. However, excessively high temperatures during their vegetative and flowering stages can be detrimental to overall plant productivity and health. Gardeners like myself in the desert southwest must also deal with very dry, windy growing conditions and intense sunshine that can be particularly damaging to our tomato plants.
Hot dry weather is normal here during the New Mexico summer and it is something that I must deal with in the garden. However, 2016 was one of the warmest and driest years on record. What made 2016 even more unusual for farmers was that the temperatures rose above normal early and often during the most vulnerable time for tomato plants. It was a relentlessly hot summer with many days above 90 degrees fahrenheit during May and June, some surpassing 100 degrees. Even if you live and garden in a more humid region, like the mid-atlantic or south, you and your garden will still be challenged by excessive early heat.
Here are a few things to consider when it comes to heat and tomato plants:
Young Plants Are Vulnerable
Established tomato plants can handle periodic hot days, but when we experience sustained temperatures in the upper 90s and low 100s, young tomato plants can become severely stressed – first as they are trying to get established, and then during their first phase of flowering. Add to that several days of low relative humidity (RH), wind, minimal cloud cover and delayed rainfall and you have a recipe for poor tomato performance.
Heat Reduces Vigor
A study from the Sudan, where the growing conditions are like those here in the Southwest, showed that when tomato plants experience severe heat stress their vegetative growth slows. This reaction is a defense mechanism resulting in a change in protein synthesis in the stems and leaves. The plants may also display visible defense mechanisms such as leaf curling and wilting. Heat stress during the plant’s vegetative stage prevents the vigorous growth required to support reproduction, disease and pest resistance; it also delays fruiting.
Plants Exhibit Poor Pollination and Fruit Set
There are several studies demonstrating the detrimental effects of heat stress on tomato pollination and fruit set. If you had healthy looking tomato plants with minimal or no fruit, the heat was most likely the culprit. The sustained heat above 90˚F in 2016 most likely resulted in fewer pollen grains produced in each tomato flower. Much of the pollen that was produced under these conditions was simply not viable. This pollen failure is associated with “alterations in carbohydrate metabolism” within the flower. If the science behind this condition interests you, here is an excellent article from the Annuals of Botany out of the United Kingdom. Some fruit set may occur in the extreme heat, but chances are the fruit will be misshapen, smaller than normal, or suffer from a hollowing effect due to the under-development of the fruit gel inside the tomato. In high humidity regions, the pollen can bind together, hindering pollination and causing fruit deformities. In dryer climates, lower RH has the opposite effect – the pollen fails to stick to the ovary.
Blossoms Drop
When the tomato flower does not produce enough viable pollen for fruit set to occur, the flowers dry out, shrivel up and drop from the plant. Blossom drop is a telltale sign that the heat is preventing pollination. During this condition, you will likely see the dried-up blossoms littering the ground beneath your plants.
Water Stress Is A Constant Threat
In addition to New Mexico’s relentless sunshine and low RH. The hotter it is and the lower the humidity, the less moist the atmosphere and thus the greater the driving force behind evapotranspiration from the soil and plants. Evapotranspiration - a measurement of water consumed by plants over time – is a combination of evaporation and transpiration. Water in the soil and on the plants’ leaves evaporates into the atmosphere and the additional moisture within plants transpires out through the stomata of the leaves. A study in the semi-arid region of Konya, Turkey to determine the effect of water stress on tomatoes found that “the highest fruit yield was recorded under the most favorable moisture conditions and the lowest fruit yield was obtained under the most severe stress conditions.”
Plants Become More Susceptible To Pests and Disease
Environmental factors, such as high sustained heat, as well as other things like poor fertility, inadequate water, dry wind, and intense ultraviolet light can stress tomato plants. My experience has been that stressed plants are more susceptible to disease and pest organisms, but there is no scientific consensus yet on whether this hypothesis is true. I did find one article noting that water-stressed plants produce fewer chemicals that deter insect feeding. (University of Illinois Extension, 2003). Our own observation on the farm is that healthy plants, especially those that get off to a good start from the beginning, show fewer problems with disease, and still bear some fruit even if they ultimately succumb to disease.
Solutions
Work on your soil. Good soil is the foundation for healthy tomato plants regardless of the conditions. Building healthy soil takes time – sometimes five years or more, so be patient. Add compost and other organic matter over time to increase your soil’s water holding capacity. Use mulch to keep the soil covered, especially in the winter. Consider having your soil tested. For a reasonable price, you can learn a lot about your soil as it stands and have a baseline and a blueprint for what amendments and improvements you must make. My most successful strategy besides compost has been the use of cover crops. A cover crop is a plant you grow for a period of time and then kill only to incorporate it back into the soil. As it decomposes, it provides organic matter and nutrients that can then be used by your subsequent garden plants. This is only one of the many benefits that cover crops bring to your garden. I have a whole blog just on this topic and it’s worth a read.
Get plants off to a good start. Buy healthy transplants with good root systems and thick stems, preferably plants that have been “born and raised” in our area. Don’t plant too early, as the plants will not really start to take root and grow until the nighttime lows stay at or above 50˚F.
Provide a wind block. Plants are most vulnerable to wind when they are young and time unfortunately this happens to coincide with spring winds. If you have a lot of wind in the spring and into early summer, put a tomato cage around your plants and wrap them with clear plastic or row cover fabric to provide wind protection.
Provide shade during the hottest part of the day. Shading has provided significant benefits to my plants, including less overall stress, increased vigor, higher yields and reduced water loss. The best scenario is to provide shade in the latter part of the day, when the sun is most fierce and the temperature reaches its daily peak. A conveniently located tree or a building is the easiest way to provide shade from the western sun. Alternatively you can build a small shade structure and use 30% shade cloth. Shade reduces evapotranspiration and lowers the ambient temperatures around your plants up to several degrees, resulting in better pollination.
Use a thick layer of mulch to encourage deep roots. Evapotranspiration happens rapidly in the first four inches of the soil. Plastic mulch is fine if you have a lot of plants, although organic mulches such as hay, shredded leaves or dried grass clippings are preferred.
Combat water stress. Make sure tomato plants get 1-3 inches of water a week (including rain). Avoid watering daily, but rather water deeply a couple of times a week to allow root systems to grow strong. One way to manage the problem of blossom end rot is to maintain an even level of soil moisture. For me, I use my finger to probe the soil around my plants. If the top three inches of soil is dry, I water. I used a drip system on a timer and through trial and error, my experience tells me how often to water using this set-up.
Combat pollination problems. Tomato flowers are self pollinating, meaning each individual flower contains both male and female reproductive parts. When blossoms emerge, mimic the work of wind and insects by gently shaking plants to spread pollen. You can also help pollination by planting flowers that attract insects among tomatoes. In dry areas, help pollen stick by misting tomato plants.
Fertilize properly. Feed tomato plants with a balanced fertilizer or tomato fertilizer every 3-4 weeks. Avoid excess nitrogen (in proportion to other nutrients). Excess nitrogen will force a lot of vegetative growth with little or no blossoms and fruit.
