Fall Is Root Season — How to Feed Trees and Shrubs Where Their Roots Actually Are

Stand under a mature tree and look at the tips of its outermost branches. Now picture a circle two to four times that wide. According to Colorado State University Extension, that's roughly how much ground a tree's root system occupies, and most of those roots sit in the top 6 to 24 inches of soil.

That's the part of the tree that's busiest in fall. It's also the part most of us never feed.

Why fall belongs to the roots

When leaves start to color and new shoots stop pushing out, it looks like a tree is shutting down. Above ground, it is. Below ground, the story is different.

Clemson Cooperative Extension puts it plainly: late summer and early fall feeding "take[s] advantage of actively growing roots when shoot growth has ceased." The Morton Arboretum explains why: when the air is cooler than the soil, new root growth is encouraged without new top growth. Soil holds its warmth long after the air turns crisp, so roots get a head start that the branches don't.

How long does that last? Oklahoma State University Extension says roots grow when soil temperature is above 40 degrees. A Vermont county forester writing in Northern Woodlands puts the minimum for root growth somewhere between 32 and 41°F, and notes that roots can grow in winter too, whenever the soil allows it, even when the air above is bitterly cold.

In most of the country, that means weeks of root growth after the last leaf has dropped. Every one of those weeks is time a tree or shrub spends building the root system it will draw on next spring.

Where the feeder roots really are

This is where good intentions usually miss. Most people feed right at the trunk, because that's where the tree is. But the roots that take up water and nutrients aren't there.

Clemson describes the root zone as the space beneath the canopy, from outside the root flare (where the trunk widens into the ground) out to the outermost branches. That's where most of the fine "feeder" roots live. Colorado State adds that a tree's roots typically extend past the drip line, the imaginary circle under the branch tips, and those small absorbing roots grow outward and upward toward the surface, where water, oxygen and minerals are most available.

So here's a practical rule: start at the drip line and work outward as far as you reasonably can. Leave the first foot or two around the trunk alone. Shrubs work the same way on a smaller scale: feed the band under and just beyond their outer edge, not the crown at the center.

How to feed the root zone organically

You don't need a deep-root injector or a trip to a garden center. You need a liquid for the roots, something slow for the soil surface, and water.

1. Drench the root zone with a liquid. MicroLife Bio-Matrix 7-1-3 ($32.99 per quart, ships free) is what we reach for here. MicroLife's own directions for root feeding are 2 ounces per gallon of water, soaking the root system well. At that rate a quart makes 16 gallons of drench. It's made with kelp, humate, amino acids and 39 strains of beneficial microorganisms, according to MicroLife, so you're feeding the soil biology around the roots, not just the plant.

For shrubs, foundation plantings and anything newly planted, MicroLife Super Seaweed ($30.99 per quart) is a gentle root stimulator. MicroLife lists 1 to 2 ounces per gallon of water as a root drench, again soaking the roots well.

A watering can works for a few shrubs. For a big tree, the root zone can be hundreds of square feet, and that's where our custom hose-end sprayer ($19.99) earns its place. The MicroLife quart bottle attaches directly to it and mixes at 80:1 as you walk. Work in slow passes starting at the drip line and moving outward, so the drench lands on the feeder roots and not in a puddle at the trunk.

2. Top-dress the soil surface. MicroLife Humates Plus 0-0-4 comes in a 9 LB jug ($35.99, ships free). MicroLife calls it concentrated compost in a bag, and its label rate for garden areas is 5 to 10 pounds per 1,000 square feet, once or twice a year. Scatter it evenly across the root zone. Clemson's guidance is that you can spread feed evenly over both mulched and unmulched ground out to the drip line, so there's no need to pull mulch back first.

If you'd rather not assemble it piece by piece, our garden and landscape bundle ($66.58, ships free) puts a set of MicroLife products for beds, shrubs and trees in one box.

3. Mulch in a ring, not a volcano. Mulch keeps soil moisture steady and slowly feeds the soil life as it breaks down. The International Society of Arboriculture recommends 2 to 4 inches: 2 to 3 inches of fine mulch, or 3 to 4 inches of coarse. Take it out to the drip line if you can, and at minimum a 3-foot radius around most trees.

The most important part is what you don't do. Keep mulch off the trunk. ISA advises pulling it back several inches so the base of the trunk is exposed, because mulch piled against the bark can lead to stem-girdling roots that slow a tree's growth or, eventually, kill it. Think doughnut, not cone.

4. Water before dormancy. A tree going into winter dry is working with less. University of Illinois Extension suggests checking soil moisture weekly by digging 4 to 6 inches down at the drip line. In a dry fall, water newly planted trees and shrubs weekly and established trees about once a month. A soaker hose laid in a loop at the drip line gives a slow soak with less runoff.

Evergreens deserve extra attention. Illinois Extension notes they don't go dormant in winter and keep losing water through their needles. Where winters get cold, water only when the air is above 40°F, and stop once the ground freezes, because roots can't take up water through frozen soil.

What to expect

Honestly, not much you can see this fall. Root growth is quiet work. The return shows up next spring, as steadier leaf-out, better color and shrubs that handle the first hot, dry stretch without sulking. Biology works on its own schedule, and that's usually a season or more.

If the lawn under those trees is on MicroLife Liquid Lawn Monthly, the Bio-Matrix in each shipment is the same one described above, and the same custom sprayer carries it.

A simple fall plan for one tree

• Pace off the drip line and mark it with a couple of stakes.
• Drench from the drip line outward with MicroLife Bio-Matrix at 2 ounces per gallon.
• Scatter MicroLife Humates Plus across the same band at the label rate.
• Spread 2 to 4 inches of mulch, pulled back several inches from the trunk.
• Check moisture 4 to 6 inches down at the drip line, and water slowly if it's dry.

That's an afternoon for a large tree, less for a row of shrubs. The payoff is what's underground come spring.

Questions about a specific tree or shrub? Call us at (281) 317-7919. We're glad to talk it through. For more on what's living in that root zone, see our Soil Biology Guide and The Soil Food Web, Explained Simply.