Written by: Greg Drecher, ISA Certified Arborist #PD-2775A
Fall doesn’t look like a busy season for your trees. The leaves are turning, growth has slowed, and it’s easy to assume they’re winding down for the year. Underground, it’s the opposite. Once the heat backs off but before the soil gets cold, trees put real energy into root growth, which is exactly why fall is one of the best windows all year to fertilize them.
Signs Your Trees Could Use It
A tree doesn’t have to look sick to be running on empty. Some of the more common signs we see on properties across Delaware County and the Main Line: smaller leaves than in previous years, canopy that looks thinner than it used to, new growth that’s noticeably slower, or a tree that’s still recovering from a stretch of dry weather, recent construction nearby, or root disturbance from grading or hardscape work. None of these are emergencies on their own, but they’re the kind of thing that’s easier to get ahead of in fall than to fix after a bad spring.
What Deep-Root Fertilization Actually Does
Deep-root fertilization delivers nutrients directly into the root zone through soil injection, rather than spreading fertilizer on the surface and hoping it works its way down. A probe injects the material several inches into the soil at multiple points around the tree’s root zone, putting it right where the feeder roots can actually use it.
That matters in Delaware County and the Main Line especially, where a lot of established landscapes sit on compacted or clay-heavy suburban soil that doesn’t drain or distribute nutrients the way forest soil does. A tree competing with turf grass for the same shallow soil layer often isn’t getting what it needs, even in a well-maintained yard. Surface fertilizer in that situation mostly feeds the grass. Soil injection bypasses that competition entirely.
Feeding the root zone directly in fall gives the tree a head start. Soil temperatures stay warm enough to support root activity well after air temperatures drop and top growth stops, so the tree keeps building root mass through the fall and into early winter. That means it goes into spring with the reserves it needs for new growth instead of starting from behind.
A Case From the Field
Last September, Greg Drecher quoted a deep-root fertilization job on a property in Lansdowne: five dogwoods, four along the street and one near the house. Our crew soil-injected roughly 100 gallons of bio-stimulant with slow-release nitrogen directly into the root zone of each tree.
These dogwoods have a longer history with us than this one treatment. A large oak on the property came down in a storm in July 2020, and at least one of the dogwoods was damaged in the process, an event Greg still remembered when this job came up again five years later. That fall, one of the dogwoods, a Kousa dogwood, was treated for fungus along with the property’s azaleas and rhododendrons. This September’s deep-root fertilization is the latest chapter in a property we’ve been caring for since at least 2020, not a one-time visit.
It’s also a straightforward example of what deep-root fertilization looks like in practice: no surface spreading, no runoff, the nutrients go where the tree can actually use them.
Winter Pruning: The Other Piece of Fall Prep
Fall and early winter are also when structural pruning makes the most sense. With the leaves down, an arborist can actually see a tree’s branch structure, something that’s much harder to evaluate in full leaf. Co-dominant stems, crossing branches, and structural weak points that are hidden all summer become obvious once the canopy clears. If you’ve been putting off a pruning consultation, the dormant season is the right time to get one scheduled, both for what it reveals and because pruning during dormancy is easier on the tree than pruning during active growth.
Why Now Is the Right Time
Waiting until spring means asking a tree to do all its recovery work at once, right when it’s also trying to leaf out and grow, at the exact moment it needs the most stored energy. Fertilizing in fall spreads that work out over the dormant season, when the tree isn’t competing for energy with new top growth. It’s the same logic that applies to tree care generally: earlier attention costs less than waiting for visible decline.
Fall fertilization is one piece of a broader Plant Health Care program that keeps trees resilient year-round.
If your trees had a hard summer, or you just want to get ahead of next year, fall is the window to do it in.
Ready to talk to an ISA Certified Arborist? Schedule your free estimate today.