Tree · Oleaceae
Green Ash
Fraxinus pennsylvanica
A tough, fast shade tree for hard sites, now living under the shadow of the emerald ash borer.
Full sun
Every 7–14 days
USDA 3–9
50–70 ft · 15.2–21.3 m
Overview
Green ash is a fast-growing native shade tree of the olive family, and it takes hold where other trees fail. Flooding, drought, wind, and alkaline soil scarcely slow it, which is why it once lined streets and parking lots across North America. Its opposite, pinnately compound leaves, each made of paired leaflets along a central stalk, and its dry winged seeds, the samaras, set it apart from the lookalike mountain ash, a rose-family tree it is not related to.
The tree climbs to 50 to 70 feet (15 to 21 m) tall under a broad, even crown, races upward while young, and burns to a clean yellow each fall. It is hardy down to -40°F (-40°C) and settles in across USDA zones 3 to 9.
There is one hard truth to carry. Emerald ash borer, an invasive beetle, has killed hundreds of millions of ash and reaches almost any untreated tree in time. Most extension services no longer recommend planting new green ash, so the better course is to care for the trees already standing and watch them closely.
Care
The short version: full sun and room to spread, young trees kept evenly moist, and a late-winter pruning to build strong structure. An established ash needs almost nothing, but watch for borer.
Light
Set ash in full sun for the densest crown and the best fall color, though it tolerates partial shade where you must. The more direct light it takes in, the stronger and more even the tree grows.
Water
Keep young trees in consistently moist, humusy loam while the roots establish, watering roughly every 1 to 2 weeks through dry spells. Once settled, ash tolerates occasional flooding and drought and rarely needs your help.
Soil
Ash is very adaptable underfoot. It grows in clay, loam, or sand so long as the drainage is reasonable, and it tolerates a wide pH from 5.5 to 8.0, acidic right through to alkaline.
Temperature
This is a cold-hardy tree, comfortable roughly from -40°F to 104°F (-40°C to 40°C) and at ease across USDA zones 3 to 9. Winter is no threat to a healthy ash.
Humidity
Outdoor ambient humidity makes no difference to ash. What counts is air movement through the canopy, since stagnant damp air invites leaf fungus like anthracnose.
Feeding
An established landscape ash rarely needs feeding. Give a balanced complete fertilizer in spring only when a soil test shows a real deficiency, never on a calendar.
Pruning
Prune in late winter while dormant, taking out dead, broken, or crossing limbs and shaping a strong structure early. The wood is brittle and prone to wind and ice breakage, so good form pays off for decades.
Repotting
none
Size & spacing
Ash matures 50 to 70 feet (15 to 21 m) tall and spreads 35 to 50 feet (11 to 15 m) wide. Site it well clear of buildings, walks, and power lines, and give that broad crown room.
When to plant
Set ash in as balled-and-burlapped or container stock in spring or fall, whenever the soil is workable. The small, inconspicuous flowers open in April and May, just before or as the leaves push out, and the winged seeds ripen through the fall.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Plant: Spring or fall; balled-and-burlapped or container stock when soil is workable
- Harvest: Inconspicuous flowers April to May before or as foliage emerges; samaras mature in fall
Hardiness
USDA hardiness zones: 3–9
Temperature: down to -40°C
Troubleshooting
One pest, the emerald ash borer, outweighs every other problem on this tree, so learn its signs first. The rest are mostly cosmetic and settle on their own. Match the symptom below, then decide whether to act or simply keep watch.
Crown dying back from the top down, with small D-shaped holes punched into the bark
Look at the bark and the leaf canopy together. The little exit holes are cut as crisp D shapes, narrow and clean, and peel back the loosened bark and you find the larvae's winding S-shaped galleries scribbled across the wood beneath. Woodpeckers chasing those grubs strip the outer bark to a pale blond flecking, and the crown empties from the top down. This is emerald ash borer, an invasive beetle whose grubs girdle the trunk from the inside, and it claims very nearly every untreated ash. A treasured tree still holding more than half its canopy, under 30 to 50 percent loss, can be carried through with a professional systemic treatment; one already heavily tunneled should come down quickly, and never move ash firewood, which is how the beetle hitchhikes.
Fix:
- Confirm infestation and assess canopy loss
- Treat high-value trees with under 30-50 percent canopy loss using a systemic insecticide on a professional schedule
- Remove and properly dispose of heavily infested or dead trees promptly because untreated ash usually die
- Do not move ash firewood, which spreads the beetle
Slow decline, with tufted witches-brooms and small, pale leaves
Watch the twigs and the leaf size. The tree thins over several years, and weak shoots crowd into dense tufts, the witches-brooms, while the leaves that do open come small, sparse, and washed pale. This is ash yellows, a phytoplasma infection living in the tree, and there is no cure once it takes hold. Steady watering and a good mulch ring slow the fade and buy a few seasons; when the declining limbs grow heavy and hazardous overhead, that is the time to take the tree down.
Fix:
- No effective cure exists
- Maintain vigor with proper watering and mulch
- Remove severely declining trees that become hazardous
Brown blotches running along the leaf veins after a wet spring
Turn a leaf over and follow the damage with your eye: irregular brown patches that trace right along the veins and margins of the leaflet, often with a curl to the blade. This is ash anthracnose, a fungus that flares up in cool, soaking springs. It tends to settle on the lower inner foliage and can shed leaves early, yet a healthy tree shrugs it off and pushes a fresh flush. Rake up and destroy the fallen leaves, thin a few limbs so air slips through, and skip the fungicide on an established ash.
Fix:
- Rake and destroy fallen infected leaves to reduce overwintering inoculum
- Prune to improve air circulation
- Maintain tree vigor with proper watering; fungicides are rarely warranted on established trees
Clumpy green-to-brown galls riding on the spring flowers
Look only at the flower clusters for this one. A tiny eriophyid mite twists the male flowers into irregular green clumps that soon darken to brown and hang on as dry, knotty masses. They cluster on the bloom and nowhere else, so the leaves stay clean and the tree's health is untouched. These are ash flower galls, and no treatment is needed; if the dry clumps look untidy to you, raking up the fallen ones is entirely optional.
Fix:
- No treatment is needed because the galls are cosmetic
- Rake and discard fallen galls if desired
Toxicity
True ash does not appear on the ASPCA toxic-plant lists and is safe around cats, dogs, and people, with no known poisoning risk to grazing animals or birds. Just do not confuse it with the unrelated mountain ash or prickly ash, which are different plants entirely.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats None
True ash sits off the ASPCA toxic list, so a curious cat batting a dropped leaflet across the floor is no cause for alarm. A large mouthful of any plant can still upset a sensitive stomach, of course.
Dogs None
Ash is non-toxic to dogs, and a chewed twig or leaf will not poison them. The bigger risk is a splinter off the gnawed wood rather than anything in the plant.
Horses None
Horses graze around ash with no trouble at all, since the tree carries no toxin known to affect them.
Rabbits None
A rabbit nibbling ash leaves or bark is perfectly safe. Bring any new green in slowly, as you would with any fresh feed, so the gut has time to adjust.
Livestock None
Cattle, goats, and sheep can browse ash foliage without poisoning, and in some regions the leaves have long been cut and stored as fodder.
Birds None
Backyard and wild birds come to no harm near ash, and many feed on the winged samaras right through fall and into winter.
People None
There is no poisoning risk to people, and the wood has been worked and burned safely for generations. It simply is not grown as a food, so nothing on the tree belongs on the plate.
Keep ash firewood out of pet bedding only for splinter and mold reasons, not toxicity.
Varieties
Most ash planted in yards are seedless male clones, chosen to skip the litter of samaras and hold a reliable shape season after season. Here are the names you will run into.
Patmore (green ash)
A seedless male green ash hardy clear to zone 2, holding an even, tidy outline and turning a clear yellow in fall.
Marshall's Seedless (green ash)
A long-popular seedless male green ash, prized for its glossy, dark green summer leaves and the absence of any messy seed.
Autumn Purple (white ash, Fraxinus americana)
A seedless male white ash clone whose fall foliage runs from reddish purple to deep mahogany and lingers on the branches.
How to propagate
Ash grows readily from seed if you have the patience for its long chill, while the prized cultivars come from grafting rather than seed.
SeedModerate
Collect samaras in fall; sow after stratification for spring germination · 1 to 2 growing seasons to a transplantable seedling
Ash sprouts readily from seed if you have patience for the long chill it demands. Gather the winged samaras in fall, then keep them in cold, moist stratification near 35°F to 40°F (2°C to 4°C) for about 90 to 120 days, and sow afterward for spring germination. Grow the young plant on for one to two seasons before it is ready to move. The named seedless cultivars will not come true this way, so those are budded or grafted instead.
GraftingDemanding
Late summer budding or dormant-season grafting · 1 to 2 years to a saleable tree
This is a nursery job rather than an easy backyard project. Named clones such as Patmore, Marshall's Seedless, and Autumn Purple are budded in late summer or grafted in the dormant season onto seedling rootstock, which keeps them seedless and true to type. Allow one to two years for the graft to fill out into a saleable tree.
Myths
Ash and mountain ash are simply two names for one tree.
The two share a name and nothing else. True ash is Fraxinus, an olive-family tree with opposite, pinnately compound leaves, while mountain ash is Sorbus, a rose-family tree carrying alternate leaves and clusters of red berries. They sit in entirely separate genera, so advice meant for one rarely fits the other.
A thinning ash crown just wants a little water and feeding to perk back up.
When the dieback starts at the very top and you turn up D-shaped exit holes with S-shaped galleries scored beneath the bark, the cause is emerald ash borer, not thirst or hunger. No watering or feeding stops it, and untreated trees usually die within about 2 to 5 years. Only early professional systemic treatment gives a valued tree a real chance.