Why have my tomatoes stopped growing? Why are the blueberries on my plants ripe but only the size of a pea? Why do I have dead spots in my lawn? Why did some of my azaleas defoliate and others have half the leaves tan? Why is my holly fern getting brown edges on the leaves?
No, these are not some cruel tricks by a mean neighbor. All these plants have something in common. They are living outside, exposed to thermonuclear radiation for at least some part of the day.
What happened to you when you stepped outside over the past couple of weeks? Yes, you started sweating almost immediately. It is how our bodies cool themselves. Plants have to keep cool, too. They “sweat” mostly through their leaves. The leaves are the hottest part of the plant, and the edges are the hottest part of the leaves. The evaporative cooling of water being transpired from the plant is necessary to keep plant temperatures within the range of life, which is the temperature that enzymes can operate. If it gets too hot, enzyme proteins coagulate just like a raw egg being broken into a hot skillet. Then, everything stops and tissues die.
As temperatures increase, the demand for water also increases. Do you not need to drink more water at 100 degrees than at 85 to maintain hydration? Of course you do — and so do plants. “But we have had rain and some heavy thunderstorms!” Yes, we have. But fast, heavy storms deliver so much water so quickly that it does not have time to sink in and runs off into the storm sewers.
How much rain fell is not how much rain was absorbed by the soil. Our sandy soils only hold about half an inch of rainfall in the top 6 inches. Most turfgrass roots are shallower than that. On bright, warm, sunny days with a light breeze, that half-inch of water is lost in only 2½ to three days, and we are back to drought. And we rarely receive natural rainfall in the right amounts at the right times, delivered in a nice, slow, steady overnight rain.
The expansion of growth in plants is powered by water pressure. If water is limiting, keeping leaves cool trumps filling fruit. Part of why the tomatoes and blueberries are small is because there was insufficient water to send to the fruit. The other part of the equation is energy — photosynthate — sugar. Every 17-degree increase in temperature doubles the energy cost of a biological process. Respiration rate — what it costs just to do routine daily business — increases with temperature. Let’s say that at 80 degrees, respiration takes 8 units of sugar and the plant is producing 16 units of sugar. It has 8 units of sugar left over after respiration to put into new roots, stems, defensive chemicals, buds, flowers and fruit. But let’s increase the temperature to 97 degrees. The respiration rate has now doubled, consuming all 16 units of sugar with nothing left over for any growth. So even if the plant did have enough water, it would have no sugar to send to the fruit.
A plant is a business. Think of sugar as profits and water as raw-material cost. Temperature is tax rates and regulatory costs. Fruit are products and dividends. The higher the temperature burden on the business, the more profits will be consumed just keeping the business solvent. Less profit is available for dividends and new-product development. If the temperature gets too high, the business downsizes as it waits for a change in business climate. If the temperatures continue to rise and the business cannot move to a better business climate — roots can be limiting at times — the business fries and closes.
There is a lot one can learn about business from a plant.