Plants have the same big challenge in summer that humans do: staying hydrated.
We all know what a wilted plant looks like, but chronic, lower-lever drought stress is less obvious. There is a progression of symptoms, as the plant tries to keep all of its leaves hydrated, fails, and sacrifices less important leaves so that new ones can grow. At this level of drought, the plants survive, but harvests are smaller because the plant has so little to spare.
These are the steps in drought stress:
If you are focused on soil nutrients, you might think your plants have a potassium deficiency. That's a reasonable guess, but backward: Potassium deficiency mimics drought stress because potassium is involved in moving water inside the plant. A plant low in potassium overheats and dehydrates. When you see the above symptoms, it's a good idea to try more water first. If you are sure your soil has been damp enough, or you see no response after a week, try supplemental potassium. (Note that leaves with severe damage won't recover. Watch the less-affected leaves and the plant's overall growth and color.)
While drought stress definitely means the plant is not getting enough water, there could be several reasons why the plant is thirsty. There are other solutions than pouring on more water. Common garden mistakes can create drought. I found many such mistakes in my own garden.
Mistake 1: I didn't start the year with enough organic matter in some of my beds. Moisture can sit between the mineral particles (sand or clay) in soil. But only organic particles—humus, compost, pieces of dead plants—can soak up water inside and hold it ready for roots to use. Obviously, you can't dig up your plants to amend the soil now, but you can add compost on top. Earthworms, water, and gravity will help put it underground—and organic compounds like humic acid will dissolve and get to plant roots.
Mistake 2: I didn't put down mulch soon enough. Organic mulches like straw, leaves, or wood chips add organic matter as they decompose. Mulches with thermal mass, like stones, condense water out of moist night-time air. Either kind of mulch prevents the soil surface from drying out and allows more biological activity in the top inch of soil, where air, moisture, and soil microorganisms meet. I am already seeing the good effects of some mulch I put down earlier this week.
Mistake 3: It's easy to have too many straws sipping out of the same cup. There's only so much water in each cubic foot of soil. If one plant occupies that foot, it gets all the moisture. If there are ten plants, they have to share ten ways. It doesn't matter if they're crops or weeds, they're all competitors. So when I didn't thin my beans, I set them up for dehydration. Give your plants the recommended spacings, or more if water is short. It is better to have 5 productive plants than 10 barely surviving, so thin now if needed, even if it means taking out mature plants.
Mistake 4: We all know that things dry quickly if we direct a fan at them. That applies outdoors as well—moving air sucks the moisture out of things, including live plants. We expanded our garden this year by taking out a fence. Without the fence, there is more wind, and things dry out. It is worth setting up temporary barriers if wind is affecting your plants.
Mistake 5: more sun than plants can handle. Creating shade can help keep the soil moist, and keep plants from overheating. Above about 90 degrees, most crop plants go into shock. Sometimes they just can't keep themselves hydrated even if water is there. Shade cloth, wooden lath, or temporary reed fencing can keep plants growing actively instead of shutting down. One index of heat stress is blossom drop in tomatoes and peppers. Others are toughness, bitterness, and tip burn in leaf crops.
Looking ahead: It is time to start fall crops, but keeping young seedlings alive and growing in hot weather can be challenging. Pots overheat in direct sun and dry out quickly. But direct-sowing is problematic too. It is often easier to monitor, protect, and water a few flats than a whole garden bed. A porch with morning sun only is ideal for seed-starting in hot climates. Or consider making a simple shade house with shade cloth, branches, or fabric. Or create a nursery bed in the garden with dappled shade, either natural or created with shade cloth. A place with bright indirect light or dappled shade, sheltered from wind and hot sun is a good place for spouting seeds, no matter how you attain it.