Is a Tree Causing Your Wet Yard, or Saving It? How Trees Affect Drainage on Durham Clay

By Kendrick Hunter, owner of Hunter Excavating in Durham, NC.

Short answer: a big, healthy tree is usually helping your drainage, not hurting it. All summer it pulls water out of the ground, and its roots open up clay that water can barely get through on its own. Trees do cause wet yards, but in a few specific ways: roots in drain pipe, leaves in gutters and drains, deep shade over turf, and roots that have slowly changed the grade. Figure out which one you have before anybody cuts anything down.

People ask this two ways. Is this tree making my yard soggy? Or, if I take this tree out, will my yard turn into a pond? We do tree work and drainage work, so we get to answer both, and the answer to both starts with what the tree is doing for the ground around it.

What a tree does for a wet yard

It drinks, a lot, for half the year

A large shade tree in leaf can move well over a hundred gallons of water out of the soil on a hot summer day. That water comes up through the roots and leaves through the leaves. Across a big canopy, that adds up to a real amount of water that never has to drain anywhere.

The catch is the calendar. A deciduous tree does almost all of that drinking while it has leaves. From late fall to spring it's mostly shut down, which happens to be when the ground is already wet and rain comes in long, soaking stretches. That's why a yard under a big oak can be soggy in February and perfectly fine in July. The oak is a big part of the reason July is fine.

Its roots open up the clay

Clay soil, especially clay that got compacted during construction, takes water in slowly. Tree roots push through it and leave channels behind when they grow and die back, and water follows those channels down. Researchers at Virginia Tech tested this in compacted clay subsoil and found that tree roots growing through it raised infiltration rates by as much as 153% compared with soil that had no trees. Urban Forestry South, run by the USDA Forest Service and Southern Regional Extension Forestry, has a short summary of that research.

It slows the rain down

Some of the rain that hits a full canopy stays on the leaves and bark and evaporates before it ever reaches the ground. The rest drips down over a longer stretch of time instead of arriving all at once, which gives clay a better chance to keep up.

It holds slopes in place

On a slope, roots hold the soil together. Clear a wooded hillside and you get more runoff, faster, and more of it carrying dirt. That water doesn't vanish. It lands on whoever is downhill, which is often the neighbor who never asked for your tree project.

Why Durham clay changes the math

Most of Durham County sits on heavy Piedmont red clay, and water moves through it slowly on its best day. I wrote about that, and about how construction compacts it, in our post on French drains vs. regrading.

Parts of the county sit on Triassic Basin soils, which are worse. The USDA describes soils like White Store and Creedmoor as very slowly permeable, with a seasonal high water table only one to two feet down. We covered what that does to yards in our post on drainage problems specific to Durham neighborhoods. For trees, it means something else: roots can't grow down into soil that stays saturated, so they spread out shallow instead.

That cuts two ways. Shallow roots are right where the water is, so those trees do a lot of their drinking near the surface. But a big tree with a shallow root plate sitting in soaked clay is easier to tip over, and any drain trench dug on that lot is going to run into roots. Both of those matter when you're deciding what to fix.

Five ways a tree can cause a wet yard

Roots in a drain line

Roots grow toward water. A perforated French drain pipe, a corrugated downspout line, or an old clay tile joint near a big tree is a reliable water source, and roots will find their way in and fill the pipe over time.

The signs are a drain that worked for years and slowly quit, a pop-up emitter at the end of a downspout line that doesn't pop anymore, water backing out at a downspout connection in a heavy rain, or a soggy strip that follows the pipe run past a tree. When a drain near a big tree stops working, roots are the first thing I check.

The fix depends on the tree and the pipe. Sometimes it's replacing the run near the tree with solid pipe and sealed joints. Sometimes it's rerouting the line around the root zone. And sometimes the drain matters more than the tree, and the tree comes out. Fast-growing, water-loving trees like willows and red maples are the usual suspects.

Leaves in the gutters, drains, and swales

Every fall, leaf drop plugs gutters, downspout lines, catch basin grates, and dry wells across Durham. A gutter that overflows at one corner dumps a whole section of roof right at the foundation, and a buried downspout line packed with leaves is the same as having no line at all.

This one is maintenance more than design: gutter guards, a cleanout at the base of any underground downspout line so it can be flushed, and grate inlets you can clear by hand. If you're putting in underground gutter drains or other yard drainage under big trees, plan for the leaves from day one.

Deep shade over grass

Under a dense canopy, the sun never hits the ground long enough to dry it. Grass thins out, the bare clay seals over, moss moves in, and the spot stays soggy. The tree isn't adding water here. It's taking away the sun and the turf that would otherwise help dry the surface.

There are two ways to deal with it. One is pruning to raise and thin the canopy so light and air get through, which is work we do as part of our tree removal and pruning service. The other is to stop fighting for grass in that spot and switch to a mulched bed or a groundcover that handles shade. What you shouldn't do is truck in topsoil to build the spot up, for the reason in the next section.

Roots that changed the grade

Big trees, especially Durham's old willow oaks, push the soil up around their surface roots over decades. That can build little dams that hold water on the uphill side of a trunk, or pockets between roots where water sits, or a slight rise that sends water back toward the house.

The tempting fix is to level it all out with fill. Don't. Purdue Extension warns that trees can decline and die from as little as an inch of clay soil added over their roots, because the roots underneath lose air. Regrading around a big tree has to be done carefully, working around the roots rather than burying them. When that won't solve it, a drain routed around the root zone usually will. Our grading services cover this kind of work.

The water that used to go into the tree

Take out a big tree and all the water it was pulling out of the ground every summer stays in the ground. A spot that was dry for years can turn soft the first summer after a removal.

There's a second piece. When we grind a stump, the hole fills with wood grindings, and as those break down the spot settles into a shallow bowl. Our post on stump grinding after tree removal in Durham covers what's left in the ground. Fill that low spot with topsoil once it settles and crown it slightly so water runs off instead of collecting. If a big tree is coming out of an area you already know stays wet, plan the drainage at the same time.

When the wet yard is hurting the tree

Sometimes the water is the problem and the tree is the victim. Roots need air as well as water, and roots sitting in saturated clay for weeks start to die back. It usually shows up top as a thinning crown and dead branch tips.

Saturated clay also loosens a big tree's grip. A shallow-rooted tree in soaked ground is the one that comes over in a storm, and our post on leaning trees covers how to tell when a tree is starting to move. That same post explains why cutting roots on one side of a tree, for a trench or a wall, can leave it leaning years later. If you're putting in a drain to help a tree, the trench has to stay out of its main root zone, or the fix hurts the tree you were trying to save.

Tree or drainage? How to tell what you're dealing with

• Soggy in winter and early spring but fine in summer, near a big deciduous tree: the tree is probably helping. Fix the water and keep the tree.

• A drain near a tree worked for years and slowly quit: suspect roots in the pipe.

• Water standing where a big tree used to be: lost water use, a settling stump hole, or both.

• Bare, mossy, soggy ground under a dense canopy all year: a shade problem. Prune, or change what you're growing there.

• Overflowing gutters and plugged drain grates every fall: leaves. Maintenance and better inlets.

• Water held on the uphill side of a trunk or pooled between surface roots: the roots changed the grade. Careful regrading or a drain around the root zone.

Most wet yards have more than one of these going on at the same time.

Why we look at the tree and the water together

A drainage contractor who doesn't do tree work can trench straight through a root zone without thinking about what happens to the tree in three years. A tree crew that doesn't do drainage can take out a tree that was the only thing keeping a backyard dry. We do both, so we look at the tree and the water as one problem and give you one plan.

Sometimes that plan is taking the tree out. Sometimes it's a drain routed around it, and sometimes it's pruning plus a little grading. And sometimes it's telling you the tree is doing you a favor and the water is coming from somewhere else entirely.

If you're replanting a wet spot

If a tree has to go, or you want a tree working on a wet corner, pick one that tolerates wet feet. River birch, bald cypress, black gum, and sweetbay magnolia all handle wet clay well. Red maple does too, but keep it well away from any drain line. A rain garden is another option for a low spot that collects water from the roof or the yard.

Questions we get about trees and drainage

Will removing a tree make my yard wetter?

Often, yes, at least during the growing season. A big tree pulls a lot of water out of the soil from spring through fall, and that water stays in the ground once the tree is gone. On heavy clay, plan for it before the tree comes down, not after.

Can tree roots clog a French drain?

Yes. Perforated pipe near a big tree is an easy target, especially for water-loving species. Solid pipe with sealed joints near the tree, or a route that avoids the root zone, keeps roots out.

Should I cut down a tree because my yard is wet?

Usually not as a first move. Most of the time the tree is helping, and the wet spot comes from grade, gutters, compacted soil, or water arriving from uphill. Find the cause first. If the tree turns out to be part of it, then it's a fair conversation.

Is it safe to regrade around a big tree?

It can be, if the work stays light over the root zone. The big risks are adding fill over the roots, cutting major roots, and running heavy equipment over them. A few inches of the wrong soil can do more damage than people expect.

What trees handle wet clay in Durham?

River birch, bald cypress, black gum, and sweetbay magnolia are good choices for spots that stay wet. Red maple works too if it's kept away from drain lines.

Do I need a permit for this kind of work in Durham?

For taking down a tree on a residential lot, almost never. The exceptions are in our post on Durham tree removal permits. Most yard drainage doesn't need one either, but work near a stream buffer or anything tying into the city's storm system is a different story.

If there's a big tree in the middle of your wet yard

The best time for us to look is right after a real rain, while the water is still sitting where it collects. We do free on-site quotes, and we'll look at the tree and the drainage together. If the tree is helping you, I'll say so.

Hunter Excavating 1904 Front St Suite 410, Durham, NC 27705 (336) 988-7585 Hunterexcavatingllc@gmail.com Mon to Fri 8:00 to 5:00, Sat 8:00 to 1:00

Veteran-owned and based in Durham, working across the Triangle including Raleigh, Cary, Chapel Hill, Hillsborough, and Mebane. More on our tree service in Durham, NC, including tree removal, trimming, pruning, stump grinding, and lot clearing.