There are oftentimes periods in the spring and summer where temperatures suddenly climb, with a 25 degree or more increase versus the previous day. This often produces temperatures exceeding 100 degrees. By the time cool weather returns, the heat wave has devastated the delicate, young tissues of plants. Commonly affected species are plants include Japanese maple, white birch, roses, bentgrass, and various flowering annuals. These plants, and most ornamentals and agronomic plant species, are what we call carbon-3 plants (C-3).
The counterpart to C-3 plants is the small group of carbon-4 plants (C-4). When temperatures begin to climb past 85 degrees, C-4 plants step up photosynthesis. You can easily spot these plants during hot weather, as they almost literally shoot out of the ground. Examples include corn, sorghum, sugar cane, pigweed, bermudagrass and kochia. This reaction to heat is what spawned the common corn farmer comment during the heat of summer, “Hear that corn a-growin’?”
Carbon-3 plants, which include the vast majority of plants, experience significant food stress in high temperatures.
However, the great majority of plants are C-3. Under normal photosynthesis, these plants will harvest carbon dioxide from the atmosphere and convert it to carbohydrates. But when temperatures rise and light intensity increases, the oxygen concentration within the photosynthesizing tissues also steps up. This slight rise in oxygen concentration causes a competition for the same plant molecule, ribulose bisphosphate, which is normally assigned to pick up carbon dioxide. The end result is a reduction of food-making and overall efficiency of photosynthesis.
As the food-making process of C-3 plants begins to drop off, food consumption (respiration) increases. That is, the rate of respiration is directly proportional to temperature. Thus, we have tension between to two opposing forces: reduced photosynthesis and increased respiration. The end result is that the supply of molecules that would be used to form sound tissues is minimized, and the resultant tissues developed during the heat wave are imperfect and of low integrity.
In addition, due to increased rates of respiration, the concomitant rate of growth of tissues increases. The roots are being called upon to not only step up their harvest of moisture, but also to increase their harvest of needed minerals. Commonly applied minerals such as nitrogen, phosphorus, and potassium are mobile elements that can move freely in the plant sap. These mobile elements can be drawn away from older tissues and translocated to young growing points, should the roots fall short in their harvesting of these elements. This is a reason why the first signs of nitrogen, phosphorus, and potassium deficiency will show up in older leaves.
Calcium and boron shortages degrade tissue strength, and increase vulnerability to heat stress.
As discussed earlier, two of the most important elements used to help form the integrity of tissues and cell membranes are calcium and boron. However, these are immobile elements. They can be slowly harvested from the soil, but once they reach their destination in tissues, they are irreversibly fixed. Young growing points cannot draw calcium or boron out of older tissues and translocate these to new growth centers.
This is why calcium and boron deficiency symptoms will always show up first in younger tissues. Furthermore, calcium, and to a less extent boron, is one of the most difficult elements to mine from the soil. Calcium oftentimes exists in the soil as an insoluble carbonate or phosphate. Roots experience great difficulty in absorbing enough, especially during a growth spurt induced by a heat wave. Thus, during heat waves, we not only have a reduction in carbon molecules (which form the building blocks for various types of tissues), but also have an induced reduction in the amounts of calcium and boron, which are necessary to cross-link important molecules in cell walls and membranes. Ultimately, the tissues developed during a heat wave are of low density and low stability. They easily succumb to heat stress, resulting in burning and scorching.
A theoretical deduction at this point, then, is that we need to minimize the shutdown of photosynthesis during a heat wave and improve the harvest of calcium and boron to help stabilize and develop sound tissues and membranes. In addition, tissue integrity needs to be maximized prior to the onset of a heat wave.
One means of accomplishing this is by maintaining balanced nutrition in plant tissues:
- It’s particularly important is to maintain a nitrogen to calcium ratio in tissues close to unity, between 1:1 and 2:1, but no higher.
- As a double check on your boron, the calcium-boron ratio in leaf tissues should be around 400:1 or 500:1, but no higher.
- If you have fruit, such as apples or peaches, you can also establish a desirable nitrogen-calcium ratio in the fruit flesh. Apples should be about 10:1, and peaches about 20:1.
- Never slug your plants with excess nitrogen, as it will suddenly shift the balances and leave calcium deficient.
Secondly, keep calcium and boron levels in the soil at levels that will provide protection during difficult times, such as during a heat wave:
- For sandy soils, try to keep calcium levels near 700 ppm, and boron near 0.8 ppm.
- For loam soils, keep calcium levels near 1,200 ppm, and boron near 1.0 ppm.
- For heavy clay soils, keep calcium close to 2,000, and boron near 1.2 ppm.
The release rate of minerals such as calcium and boron, as well as phosphorus and potassium in the soil needs to be maximized to feed the plant when large demands are made:
- Increase the microbial activity of the soil.
- Use compost and the new wave of commercially available microbial additives.
Prior to and during heat waves, spray plants with the following materials:
- Nutri-Cal Calcium—4 oz. per 2.5 gallons—Spray your prized plants in the early morning or late evening to maximize
- Liquid 10% Boron—4 drops per 2.5 gallons—Coverage and time for uptake of nutrients.
- Table Sugar—1.5 lbs. per 2.5 gallons—Spray ahead of and during the heat wave.
- Ivory Dish Soap—2 drops per 2.5 gallons—Spray at 5-day intervals. The key is to stay ahead of the stress.
- B-Complex Vitamin—4 tablets per 2.5 gallons—Crush and dissolve in water. Maintain adequate irrigation, but do not waterlog plants, especially in the heat.