Throughout the year in a nursery, trees and shrubs are pushed to maximize growth more than an average plant in the landscape. In the spring we apply a slow-release granular fertilizer in the field, fertigate through our irrigation system, and we are trimming the trees twice during the growing season. The practice encourages new growth and more branching in the plants. This is done do stimulate two years of growth in only a single year, but it is important to know when to stop.
You can see in the photo below this honey locust was trimmed earlier this summer and has since put on about 12″ of new growth on the new branch. If we were to continue to fertilize and trim this tree, we could push new growth until late fall. The problem with doing this is that all the new growth is tender and green. This late season growth will typically die back from our cold winters.
To prevent this die back from occurring takes a bit of planning throughout the year. The slow-release fertilizer we apply in the field is designed run out by August. We stop injecting fertilizer into our irrigation system and make sure we have all our summer trimming done by August 15th as well. This allows the tree’s growth to become woody; we call this process hardening off. Once bark has formed on the growth it has a much higher chance of surviving the winter and you won’t have a big setback come spring.