Photo: Kristine Paulus, CC BY 2.0 · Wikimedia Commons

Swamp Cottonwood

Populus heterophylla

Also called swamp poplar, downy poplar, river cottonwood, Populus argentea.

A big poplar for swamps and sloughs, with heart-shaped leaves up to 8 inches long that open in white down, rare at the north end of its range.

Tolerates wet soilRare

Type: Large tree
Mature height: 70–90 ft
USDA zones: 6–9
Growth rate: Medium
Sun: Full
Soil moisture: Average, Wet
Grows wild in: Floodplain forests, stream banks, swamps and wet flats
Native range: 24 states, north to Michigan, south to Florida and west to Missouri
List all 24
Alabama, Arkansas, Connecticut, Delaware, Florida, Georgia, Illinois, Indiana, Kentucky, Louisiana, Maryland, Massachusetts, Michigan, Mississippi, Missouri, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, South Carolina, Tennessee and Virginia
See what is native in your state

Hardiness zones Find your zone by ZIP code

Swamp cottonwood is the poplar of the wettest bottoms, growing on sites too wet for eastern cottonwood. Despite its name it is not a true cottonwood but belongs to a group of poplars with round leaf stalks. It is uncommon in most of its range, rare in tree collections and hard to grow from cuttings. Connecticut, Massachusetts and Michigan list it as endangered, and New York lists it as threatened. Plant it at a swamp edge, in a wet swale or on a pond margin where water stands through the spring.

Size, growth and what to expect

Swamp cottonwood grows at a medium rate, about a foot and a half a year in our growth estimator. On poorly drained ground, height growth has averaged about 2½ feet a year over 30 years. Expect 70 to 90 feet. It grows little after 40 or 50 years and seldom stays sound past 80.

Catkins open from March to May, before the leaves. From April into July, female trees shed tiny seeds on tufts of cotton that ride the wind and float on floodwater. The leaves, up to 8 inches long, open coated in white down.

It grows wild in swamps and wet bottoms from Connecticut to northwestern Florida and Louisiana, and up the Mississippi Valley to Missouri, Ohio and southern Michigan. In 2003 it was discovered in southern Ontario, at that time the most recently found Canadian tree.

How to grow Swamp Cottonwood

Swamp cottonwood needs full sun, since older trees will not stand shade. It is most often found on heavy clay and grows best in the deep, moist soil of shallow swamps. Give it a slough, a swamp edge or a low flat where the water table stays near the surface for all but two or three months a year. Poplar roots run shallow and far, so plant it 30 feet from a wall and more than 50 feet from overhead wires, and set trees 40 feet apart.

Saplings ship bare-root or potted. Plant them dormant, from leaf fall until bud swell, in a hole wider than the roots and no deeper. Set the root flare at the soil line, put the native soil back in, and keep the mulch clear of the trunk. The cottony seed lives only a week or two, so sow it the day it arrives. Scatter it on the surface of wet, bare soil, uncovered, and keep the bed moist until the seedlings are up.

Water weekly through the first summer unless the ground stays wet. Rabbits and deer browse seedlings and young trees, so cage them for the first few years. Prune only to keep one leader, in the dormant season.

Trees are male or female, and only females bear the cottony seed, from about 10 years old. No pest or disease is recorded for swamp cottonwood alone, but those of eastern cottonwood probably attack it. They include the cottonwood leaf beetle, borers, leaf rust and cankers. Where eastern cottonwood grows nearby the two can cross.

Heights and zones are typical ranges for the species in cultivation; local conditions and seed source shift them.

Sources (14)