We have had some doozy hot, dry days this summer already. Remember that almost two week stretch of consecutive fry-‘em-up 90 plus days way back in June/July? It caused some severe stress on plants that is showing up now in overall plant performance. Long-term wilting is never good for plants, though some handle it better than others.
Have you ever noticed how hydrangeas notoriously wilt during the day but seem to perk back up as the day cools toward sunset? What is going on there? And just because my plant is wilting does it mean I need to water it? I shall attempt to answer your burning…or wilting questions with as much wit and dry humor as possible.
Water loss from plant leaves is called transpiration. Plants primarily gain water from the soil via their roots, as I am sure you remember from elementary school. Leaves wilt on a hot, dry day because the demand for water outpaces the roots’ ability to supply it. One of the biggest factors affecting the rate of water loss is the relative humidity near the leaf surface. A plant is less likely to wilt on a hot, humid day because the difference between humidity inside and outside the leaf is much less, and we all remember how nature’s rate of moving stuff around from highs to lows is based on how great the difference.
Wilt stress is not good for plants in the long run because they can enter a dangerous stage called photorespiration (a.k.a bad plant news). When leaf stomata close up to preserve what little moisture is left in the leaf, no carbon dioxide enters and the plant is no longer able to make its carbon based food source efficiently. It can even start burning up what food reserves it has faster than it can photosynthesize. Chronic wilting can starve a plant to the point it can not fight off pests, pathogens or your dog’s special attention.
When you see that poor leaf dropping, your first instinct might be to grab a water source to hose that sucker down. However, I say not so fast. I would first ask myself, “Self, why are those leaves not getting enough water?” (Self asks some pretty good questions). There could be something going on in the land down under of which you are not aware. The best plan is to take a trowel and go dig around the plant’s roots and feel the soil moisture. Your plant might have plenty of water, but is not able to take it up for some reason. The plant could be suffering from root rot (moisture sticks around too long) or roots were damaged during transplanting or construction. More water might not be your answer. Changing the plants environment through relieving compaction, mulches and compost applications, and watering deeper and less often. There are several ways to cool off those panting plants including sprinkling them in the afternoon with a bit of water.
So, the next time you see your megaphylls droop, do a little checking before you give them a drop.
Josh Steffen, Horticulture and Facilities Manager