   Cottonwood Trees - October 19, 2011 Jeff Schalau, Agent, Agriculture & Natural Resources University of Arizona Cooperative Extension, Yavapai County Yavapai County has two native species of cottonwood trees in addition to the native quaking aspen trees. The cottonwood trees are Fremont cottonwood (Populus fremontii) and narrowleaf cottonwood (P. angustifolia). These two trees also hybridize to produce Populus xhincleyana. Aspens are in the same genus: Populus tremuloides. Other cottonwoods species and hybrids have also been introduced. The most common of these is the Lombardy poplar (P. nigra). Fremont cottonwoods can become very large when grown in or near a perennial water source. When young, they often perform well in irrigated landscapes. Over time, they usually outgrow the irrigation system and begin to die back or succumb to disease. I have rarely seen large cottonwoods that have survived longer than 10 or 12 years in a landscape unless they have a creek, river, ditch or septic system to sustain them. When regularly flood irrigated, they will grow and survive, but most drip systems will not provide adequate irrigation. When they survive and grow, cottonwoods often outgrow their allotted space in the landscape and require some radical pruning. This often is a severe “topping” of the tree (topping is not a recommended pruning practice – go to the website and see the Backyard Gardener dated December 31, 2003 for more information). Lombardy poplars were also once very common in the retail nursery trade but have fallen from favor for various reasons – the primary one being disease susceptibility. These European natives exhibit a columnar or pyramidal growth form and were often used in hedgerows and windbreaks. I have never seen narrowleaf cottonwoods for sale in nurseries, but I have several of them in my yard in Prescott. These natives are somewhat spindly and keep regenerating along the creek going through my property. They are also short-lived, but are taller and skinnier than Fremont cottonwoods. Acorn woodpeckers love to nest in their dead trunks. I don’t mind this, but it is not common for people to want dead trees in their landscape (I’m just crazy I guess). Aspens are loved for their white bark, trembling leaves, and vivid yellow fall color. However, they should only be planted at higher elevations. They struggle a little in Prescott (at 5,500 ft. plus), but when planted at lower elevations, they perform marginally or poorly. In Oak Creek Canyon they would be worth a try. Flagstaff is a better location for them, but there natural stands are experiencing “aspen decline”. In the Flagstaff area, aspen decline has been a problem in native aspen stands following a June freeze in 1999, subsequent droughts in 2001-02, tent caterpillar damage after that, and constant grazing pressure from elk. Many aspen stands in the Flagstaff area have died back severely. Cottonwoods, poplars, and aspens can also produce suckers from the root system. These suckers sprout from adventitious buds and have the ability to produce new trees if left alone. In irrigated landscapes, suckers are often undesirable and require additional maintenance. When removed, a wound is also left behind creating an entry point for disease organisms to establish themselves. Species of Populus are widely used in landscapes – especially when fast growth is desired. Most of these are hybrids and most often they are so-called “cottonless” cottonwoods. These are male clones selected because they do not produce the cottony seeds that become a nuisance. This is not to say they are a good choice for residential landscapes – they are not. I’ve mentioned the cottonwood’s susceptibility to diseases above. Some specific fungal diseases are sudden death from cotton or Texas root rot (Phymatotrichopsis omnivore) and slower acting rot diseases caused by Cytpospora canker (Cytospora chrysosperma), Ganoderma root rot (Ganoderma lucidum), and Inonotus heart rot (Inonotus munzii). Cottonwoods are also very susceptible to true mistletoe infections. So, you’ve read this far – do you get the idea that I don’t recommend cottonwood trees for residential landscapes? That’s right! I think they are great along the rivers and creeks where they are part of a native ecosystem, but, being native, they also have their own set of natural population control mechanisms that are constantly keeping them in check. Follow the Backyard Gardener on Twitter – use the link below. If you have other gardening questions, call the Master Gardener line in the Camp Verde office at 928-554-8999 Ext. 3 or e-mail us at cottonwoodmg@yahoo.com and be sure to include your name, address and phone number. Find past Backyard Gardener columns or provide feedback at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.  |