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Native trees and shrubs for heavy clay: which take it wet, which take it dry, and how to plant them
Which native trees and shrubs grow in heavy clay, why floodplain trees cope best, how to test your drainage with a bucket, and how to plant them in clay.
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Why the trees of river bottoms take clayWhat clay does to rootsA spade-and-bucket test for drainageNative trees and shrubs that take clayPlanting in clayWhat fails in clayWhere to startCommon questionsSourcesFor heavy clay, the surest native trees are those of river bottoms, among them overcup oak, black gum and American persimmon. Their home ground floods in winter and spring and bakes hard in summer, and they take both. Shrubs at home on wet and dry ground alike, such as nannyberry, gray dogwood and arrowwood viburnum, take clay too. Where a test hole still holds water the next morning, only the wet-ground plants belong. Plant wide and shallow, and backfill with the clay you dug.
Why the trees of river bottoms take clay
The low backwater flats of southern river bottoms are often heavy clay that floods in winter and cracks in summer. Overcup oak grows on that clay, and Nuttall oak on flats under 3 to 8 inches of water all winter. Water hickory takes spring floods and a summer that cracks the clay.
Overcup oak holds that ground by standing early floods that kill the trees which leaf out sooner, not because it needs the wet:
“It is frequently a lack of competition rather than an affinity for the backwater sites that allows this species to dominate.” (USDA Forest Service, Silvics of North America, Overcup Oak)
Given better ground, these trees take it, and Nuttall oak does best in rich, moist loam. How they live through floods is the subject of how floodplain trees survive high water.
What clay does to roots
Clay particles are the finest in soil, under 0.002 millimeters across, and the tiny pores between them take water in slowly and hold it tightly. After a soaking, water fills the pores and drives out the air, and roots short of oxygen stop working and rot. Dry, the same clay shrinks, cracks and turns as hard as brick. It packs down easily too, most of all when walked on or dug wet. Packed clay stops water, air and roots.
On heavy, wet clay, roots stay shallow. Overcup oak starts with a taproot but ends with a shallow, saucer-shaped root system on the clay flats where it grows.
A spade-and-bucket test for drainage
Soil that drains sheds a soaking rain within a day, while clay that is packed or low-lying can hold it for days. The University of Maryland Extension’s percolation test tells which you have, with a spade, a bucket and a ruler:
- Dig a hole a foot deep and a foot across.
- Fill it with water and leave it overnight.
- Refill it the next day and time the drop.
Water gone within eight hours means clay that drains, and a drop of less than an inch an hour is too slow. If the first filling still stands in the hole the next morning, the clay stays wet, the wet ground of our planting-site check.
Native trees and shrubs that take clay
Every plant in the two tables is in our shop and takes clay. The moisture column uses the site check’s scale, from wet ground that puddles after rain to dry ground that dries within a day.
| Tree | Height, feet | Zones | Soil moisture |
|---|---|---|---|
| American basswood | 60–90 | 3–8 | Average |
| American persimmon | 35–60 | 4–9 | Wet, average, dry |
| Black gum | 40–70 | 4–9 | Wet, average, dry |
| Common hackberry | 40–60 | 3–9 | Average, dry |
| Delta post oak | 60–80 | 7–9 | Wet, average |
| Downy serviceberry | 15–25 | 4–9 | Average |
| Eastern redbud | 20–30 | 4–9 | Average, dry |
| Northern catalpa | 40–70 | 4–8 | Wet, average, dry |
| Oglethorpe oak | 50–80 | 6–9 | Wet, average |
| Overcup oak | 40–60 | 5–9 | Wet, average |
| Shagbark hickory | 60–90 | 4–8 | Average |
| Washington hawthorn | 20–30 | 4–8 | Average, dry |
| Shrub | Height, feet | Zones | Soil moisture |
|---|---|---|---|
| American beautyberry | 3–6 | 6–10 | Average, dry |
| Arrowwood viburnum | 5–10 | 2–8 | Wet, average, dry |
| Blackhaw viburnum | 12–15 | 3–9 | Average, dry |
| Common prickly ash | 10–20 | 3–8 | Average, dry |
| Devil’s walking stick | 10–20 | 4–9 | Wet, average |
| Gray dogwood | 4–10 | 4–8 | Wet, average, dry |
| Nannyberry | 12–18 | 2–8 | Wet, average, dry |
| Northern bayberry | 5–10 | 3–7 | Wet, average, dry |
Planting in clay
Dig when a handful of the clay, squeezed into a ball and bounced in your palm, breaks into crumbs.
Make the hole two to three times as wide as the root ball or the spread of the roots, and no deeper than the ball measured from the root flare. Leave the bottom undisturbed, slope the sides, and notch the round wall with the spade so roots can get out into the clay.
Set the root flare at grade where the clay drains. Where water stood in the test hole overnight, set the ball 2 to 4 inches high and bring soil up to its sides, not over its top.
Backfill with the clay that came out, its clods broken up, and add no compost, topsoil or sand. Compost makes a pocket that holds water the clay around it cannot drain, and the roots stay in it:
“Roots often won’t grow beyond the well-aerated, enriched planting hole soil into the surrounding clayey soil.” (University of Maryland Extension, Planting a Tree or Shrub)
An amended hole is the ninth of our ten planting mistakes. Organic matter, where a plant wants it on clay, goes over the whole future root area.
On clay the watering calculator schedules a watering daily for two weeks, then every 3 to 4 days to week 12 and every 7 to 10 days after. Before each watering it has you check the soil a few inches down, since too much water at once waterlogs the planting hole.
What fails in clay
Azaleas and rhododendrons want acid soil that drains, and in wet clay they fall to Phytophthora root rot, a water mold of poorly drained ground. Pinxterbloom azalea needs a raised mound or another spot on heavy clay. Mountain laurel fails in clay, and wet clay rots the roots of sassafras.
Chestnuts fail too. Allegheny chinquapin needs ground free of heavy clay and standing water. American chestnut dies of Phytophthora root rot on clay and low ground, mainly in the South. Shellbark hickory takes a few weeks of spring flooding on deep loam but does poorly on heavy clay.
Where to start
Run the bucket test first, since it decides which rows of the tables fit your ground. Then use our tools:
- Planting-site check: open it set to wet if the hole held water overnight or average if it drained, then add your zone and sun for a verdict on any species.
- Tree finder: the species we grow for wet ground, by zone, size and sun.
Blue Whale Trees grows every plant in the tables from seed. The three oaks are among our rare and uncommon oaks, and the shrubs among our native shrubs.
Common questions
What trees grow in clay soil with poor drainage?
Overcup, Oglethorpe and Delta post oaks, black gum, American persimmon and northern catalpa take clay that stays wet. Nuttall oak and water hickory do too.
What shrubs grow in clay soil and shade?
Arrowwood viburnum, gray dogwood and nannyberry take clay in sun or shade, wet or dry. Blackhaw viburnum takes shade too but wants clay that drains.
Should I add compost or sand to a planting hole in clay?
No: compost holds water the clay around it cannot drain, and the roots stay in that pocket. Fine sand fills the pores of clay and slows drainage further.
How can I tell if my soil is clay?
Moisten a lump of soil, knead it like putty and push it between thumb and forefinger into a ribbon. A ribbon 2 inches or longer before it breaks means clay.
Sources
- 1. Soils & Plant Nutrients: NC State Extension, Extension Gardener Handbook: clay particles under 0.002 millimeters, water entering clay slowly and held tightly in small pores, a day for soil that drains to shed a soaking, root rot in airless soil, clay too hard dry to work, compaction when wet, sand that lowers pore space, and the ribbon of the feel method.
- Common Soil Problems: University of Maryland Extension: poor drainage in clay, low ground and packed soil, water driving air from the pores, clay brick-like when dry, the percolation test with its eight hours and inch an hour, the ball test before digging, fine sand that slows drainage, and compost in a planting hole that waterlogs it.
- Planting a Tree or Shrub: University of Maryland Extension: a hole no deeper than the root ball measured from the flare and at least twice as wide, organic matter in the hole keeping roots from the clay around it, and soil improved over the whole future root area rather than the hole.
- How to Plant Trees and Shrubs: The Morton Arboretum: a hole two to three times as wide as the root ball and no deeper, an undisturbed bottom, sloping sides, and backfill of the soil that came out with no amendments.
- Planting Trees Correctly: Clemson Cooperative Extension: notching the sides of the hole in heavy clay, a ball set 2 to 4 inches high in poorly drained soil with soil built up its sides and none on top, and clods broken before backfilling.
- Azalea & Rhododendron Diseases in South Carolina: Identification, Prevention, and Treatment: Clemson Cooperative Extension: Phytophthora root rot of azaleas and rhododendrons, a water mold that thrives in poorly drained or wet ground.
- Overcup Oak: USDA Forest Service, Silvics of North America: poorly drained clay flats of southern river bottoms, a seedling taproot giving way to a shallow, saucer-shaped root system on heavy clay, and early floods rather than preference behind its hold on backwater sites.
- Nuttall Oak: USDA Forest Service, Silvics of North America: heavy, poorly drained clay in the first bottoms, and flats under 3 to 8 inches of water through the winter.