For this project we have focused on four main species so far: Fremont cottonwood, velvet mesquite, coyote willow and Goodding’s willow, as well as a pilot planting of screwbean mesquite. For the cottonwood and willow trees we use a technique called vegetative propagation to grow them. A section of a branch from the newest growth of a tree called a cutting is collected in the winter when the tree is dormant and placed into soil with the addition of some rooting hormone on the base of the cutting. In this way, the lower inches of the cutting develop roots, while the top part of the cutting makes leaves from dormant buds. Trees can grow like this in the wild too, botanists refer to wild vegetative propagation as layering. Our cuttings are kept in a cool environment and watered daily to encourage root growth. Once the cuttings have both leaves and roots, we water them every other day so that their roots are able to dry down and then become saturated, mimicking the ebb and flow of the bank of a river system.Adair Patterson, greenhouse manager at NAU
For the mesquite species we grow from seed that is collected in the fall. Once we clean the seed from the pods, a process called scarification is necessary to break through the seed coat and allow the seed to germinate. In the wild, this occurs through animal digestion or weathering over time. In our case, we use sandpaper to break through the tough coat. Seeds are then soaked overnight and planted the next morning. Mesquite grows notably slower than cottonwood and willow, it takes them about three months to reach 1-2 feet in height, whereas a willow can grow about a foot in a couple of weeks given ideal conditions.
Once any of the above species have filled out their pot with roots, we either transplant them or take them outside for hardening. Hardening a plant involves decreasing it’s water intake and exposing it to natural outdoor conditions in preparation for moving it to the field. As for transplanting, this is the technique we use for growing in the three foot long pots. First we grow the tree in a shorter pot, then transplant into the long pot. Growing long pot trees has been an interesting challenge over the past few years. The tree might look large above ground but the roots might not be fully developed in the soil. Obviously, the more roots they have when they get to the field, the better chance they have to establish. We therefore have developed a set of techniques to help them increase root development. We give them specific nutrients designed for root growth and keep them in a relatively cool greenhouse to slow their leaf growth but encourage their roots. In this way, they fill out their pots more successfully.
Growing the four species is important because they each have different adaptations and perform different roles in their environment. For instance, the mesquite are well adapted to dry conditions, so they can fill out the top of a bank where the water table is lower on average, while the cottonwoods and willows can be planted closer to the water table and help to stabilize banks with their faster rate of root growth. The longer pots allow the roots to reach the water table faster and can be placed both further from the bank and used as primary establishment to support the other smaller pots. All four species have different birds, insects and other wildlife that benefit from them. I think this four tree design is really unique and interesting in restoration, I haven’t seen it used much with other projects.