Yes, tree roots grow fall months when soil stays above 40°F (4°C) at root depth. This shocks many gardeners who think dormancy starts when leaves drop. Roots keep working down below long after the parts you see shut down for winter.
I dug up a fall-planted maple the next April to check its root growth. The old root ball showed dozens of bright white root tips pushing out into the native dirt. These new roots had grown four to six inches past the nursery pot during fall and early winter. That underground spread explained why the tree leafed out so strong that spring.
I ran this same check on three more trees the following year. All of them showed the same pattern of new white roots reaching outward. This hands-on test proved to me that fall planting gives trees a real head start. Now I always aim for October when I have the choice.
Fall root growth happens due to a gap between air and soil conditions. Cool air temps reduce how much water leaves and branches pull from the tree. Warm soil below keeps roots active and growing. Trees shift energy away from top growth and send it down to the root system during this change of seasons.
MSU Extension research backs up what I saw in my yard. Soil temp is the main driver of root growth. Air can drop to freezing while soil four inches down stays at 50°F (10°C) or warmer. This heat lag gives roots extra weeks of growth time after the first frosts hit. Fall soil stays warm longer than most folks expect.
Autumn root development gives newly planted trees real perks. Roots that spread before winter tap into larger areas of soil for water and food. This bigger root system fuels stronger spring growth when leaves come out. Trees with fall-grown roots show better drought handling during their first summer than spring-planted ones do.
Root growth dormancy kicks in once soil temp falls below the threshold. Different types have slightly different floors, but 35-40°F (2-4°C) marks where most roots stop working. True dormancy lasts until spring warmth brings soil back above these marks. The tree is not dead during this time. It just waits for better conditions.
Track soil temp through fall to learn your local root growth window. Push a probe four inches deep in the morning before sun warms the surface. Write down readings each week starting in September. You will learn how long roots can stay active in your zone. This data helps you time fall planting for the best results.
The window for fall root growth closes four to eight weeks after the first hard frost in most northern areas. Southern gardeners get much longer windows. Their root growth can stretch into December or even January. Knowing when your soil drops below the growth mark tells you the latest safe date for planting.
Read the full article: When to Plant Trees for Best Growth
