Desert Blooms

Southwest Yard & Garden by Dr. Marisa Thompson These cottonwoods in Albuquerque skipped the glorious golden stage of fall color altogether. 2018. Photo credit Wes  Brittenham. Question: Why are some cottonwood trees turning brown rather than yellow this fall? Sometimes a portion of the tree (often the lower portion) turns brown while the crown does turn yellow. I think I have seen this in other years, but this fall it seems more pronounced. Is it weather related? Moisture? -         Wes B., Albuquerque, NM Answer: Explanations for why leaves change color the way they do can be related to the species or cultivar, temperature fluctuations, seasonal day length changes, and potentially the soil moisture levels too. A general rule is that while temperature tends to affect the intensity of leaf color, it’s the shorter days and longer nights that trigger overall color change. Chlorophyll is the green pigment that starts the process of...

Southwest Yard & Garden by Dr. Marisa Thompson Tree growth is slowing as temperatures fall (photo credit M. Thompson). Question: I’ve seen conflicting advice online on whether or not to fertilize trees now to promote healthy root growth. -         Camille R., Albuquerque Answer: Don’t fertilize anything in fall because we want growth to slow with dormancy, and the salts in fertilizer will either just sit there and be unhealthy for soil/roots or actively damage roots. When carefully selected and applied, fertilizers can help boost a plant that’s already putting on a flush of top growth, like in the spring and early summer. In our areas, applying fertilizer now may extend late-season growth, and that new tender growth is particularly susceptible to cold injury. Water, applied low and slow to the whole root zone, is the best “fertilizer” to help trees through the winter, plus a cozy, fibrous mulch layer on top of the soil. Depending...