The browning of giant sequoias, Sequoiadendron giganteum, around the area has people worried, wondering if their trees are dying. It is not uncommon in dry winters for these beautiful trees to turn reddish-brown in the winter. I have always thought the cause was lack of winter moisture and too much winter wind and sun. This has been a fairly wet season for us and I didn’t expect to see such discoloration this year.
I asked University of Nevada Cooperative Extension’s horticulturist, Wendy Hanson, why she thought the trees were suffering this year even with good precipitation. Wendy said “Some trees are browning and others aren’t. It seems to me that trees that were watered thoroughly through the summer over their entire dripline are doing better than those watered with a few drip emitters up close to the trunk. I think it has to do with underdeveloped root systems. Those with only a few emitters trees are unable to grow a widespread root system to absorb the water even if it’s there.”
Roots are amazing. For example: at the end of it’s first year’s growth, an apple tree can produce as many as 17,000,000 root hairs with a total length well over a mile. It will only do this if the soil is rich and there is water, nutrients and oxygen available. We plant sequoias, which are native to the western slopes (wetter, more humid and better soils) of the Sierra Nevada, outside their range in our arid desert soils and expect them to thrive. They are very adaptive if we irrigate them properly.
About 90% of a tree’s roots are to be found in the top 18 inches of the soil and radiate out from the trunk, three to four times the height of the tree. People plant trees, put in three or four emitters near the trunk and leave them that way, rarely expanding the area watered. This will not allow the tree roots to develop as widely as they should. With fewer roots to absorb water, the tree can easily suffer drought and climate stress. I suspect this is why sequoias are browning even in a wet year; they weren’t properly irrigated during the growing season. They didn’t develop the feeder roots they needed to absorb the amount of water necessary for good plant health.
Adopt a wait and see attitude. The trees should be all right and get their healthy color back as the season warms. Don’t fertilize them until they start looking good again. Water out to the dripline to a depth of 18 inches all summer and fall.