Cottonwoods leaves are drying up and falling off
It's late July and we have been in the mid 90's for weeks. My cottonwoods are 1-4 years old, they are planted in a row for a windbreak and run on a drip system. I water them every other day for an hour, so they each get 2 gallons of water every time they are watered. They were fertilizer spiked in spring, now the leaves on about a forth of them are drying up and falling off. Are they getting to much or not enough water? I also have a row of blue spruce that get everything the same and about 4 of them are loosing needles and turning brown.
We have been having extremely hot temperatures lately. The extreme heat makes the leaves of plants, including trees, lose a lot of water to evapotranspiration. That is the process of water traveling out of the soil, into the roots and up the trunk out of the leaf to ensure a continuous flow of water into the leaves. If the supply of water is not sufficient to keep up with that flow, the leaves will wilt and eventually the damage will lead to dead brown edges and eventually to dead leaves that the tree sloughs off to minimize the damage to the leaves and not to twig and branch tissue.
Two gallons of water was probably sufficient for small seedling trees in most summer conditions but not for older trees and not in the conditions of late. The 4 year old trees are probably double the size of the 1 year old trees. Trees are made up of approximately 80 to 90 percent water and when that supply is evaporating into the environment around the tree, it leaves the tree wanting more. On top of that basic description, cottonwoods are riparian trees that should have their root systems soaking in water in natural conditions. I would suggest doubling the amount of water given to the trees at each watering or watering more frequently to keep up with the moisture loss in the heat.
The use of fertilizer encourages new growth and that new growth requires moisture to sustain it, as well as diluting the nutrients in the soil to prevent them from becoming toxic to the plant roots. So the fertilizer encouraged a larger tree and a larger tree requires a bigger drink of water.
Blue spruce are native to the high Rocky Mountains and receive upwards of 12 feet of snow in a given year. The larger and larger they get the more water they will require to maintain a healthy canopy of needles to sustain the growth of new tissue.
I hope this helps,