Our intense summer heat has a tremendous effect on how and when we fertilize our plants. Labels on fertilizers provide application rates and schedules that are based on the average temperatures and the average length of the growing season for most of the country. There's nothing average about our high summer temperatures or our very long, six month, summer growing season. We need to adjust fertilizer application rates and timing throughout the growing season as temperatures fluctuate, from April until October.
We can have hot spells as early as April that cause stunted growth of new transplants as well as flower drop on young tomatoes and scorched buds and leaves on all plants. Many gardeners respond to the damage by increasing fertilization, thinking that the plants need a nutrient boost that will encourage renewed vigor and the replacement of fallen flowers and burned leaves. In fact, the opposite is true. Do not fertilize plants at all during spring hot spells and then, when temperatures have moderated, feed lightly, at (generally) half the recommended rate until there are visible signs of new growth.
The latter part of May and the first two weeks of June are periods of rapid plant growth and rapid ripening of fruits and vegetables. Daytime temperatures in the 90s and nighttime temps in the low 60s are fairly moderate (to us! — gardeners in other parts of the country would consider them really hot). Flowering plants including summer vegetables do need regular feeding at the recommended rate during these weeks.
When temperatures are consistently above 95 degrees during the day and in the high 60s or low 70s at night, plant growth slows significantly. Plants enter a state of semi-dormancy that helps ensure their survival during extreme heat. In July and early August, when temperatures are at their highest, often over 100 degrees for days at a time, there will be little growth and little need for fertilization. Feeding semi-dormant plants in July will actually stress them. Resume feeding at half the recommended rate as soon as nighttime temps have cooled down into the 60s. Cooler (and slightly longer) nights give heat stressed plants time to recover.
Fertilizer formulations also affect plants during hot weather. Liquid fertilizers are often formulated with high percentages (above 10%) of the macronutrients nitrogen, phosphorus and potassium. That's because liquid fertilizers are washed out of the soil by irrigation and must be reapplied more frequently, usually every two weeks, to maintain consistent levels of nutrients. And higher percentages of nitrogen especially force rapid growth of green tissue. Granular fertilizers generally are formulated with lower percentages (below 10%). They break down more slowly in the soil and can be applied less frequently. The lower levels of nitrogen in granular formulations keep green tissue growth at a steady pace.