Tree · Oleaceae
Green Ash
Fraxinus pennsylvanica
A rugged, quick-growing shade tree that thrives on difficult ground, though its future now hinges on 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 in the olive family (Oleaceae) that establishes where fussier trees fail. It withstands flooding, drought, wind, and alkaline soil, which is why it once lined streets and parking lots across much of North America. Two field marks separate it from the lookalike mountain ash, a rose-family tree it is not related to: its opposite, pinnately compound leaves and its winged seeds, called samaras.
Mature trees reach 50 to 70 feet (15 to 21 m) tall with a broad, even crown, grow quickly while young, and color a clean yellow in fall. The species is hardy to -40°F (-40°C), performing across USDA zones 3 to 9.
One caveat governs everything else here. The 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 sound approach is to care for the trees already standing and watch them closely.
Care
The short version: give it full sun and room to spread, keep young trees evenly moist, and prune in late winter to build strong structure. An established ash needs almost nothing, but watch for borer.
Light
Full sun produces the densest crown and the best fall color. Ash tolerates partial shade, but more direct light builds a stronger, more even tree.
Water
Keep young trees in consistently moist, humusy loam while their roots establish, watering roughly every 1 to 2 weeks during dry spells. Once established, ash tolerates occasional flooding and drought and rarely needs help.
Soil
Ash is very adaptable underfoot, growing in clay, loam, or sand provided drainage is reasonable. It tolerates a wide pH from 5.5 to 8.0, acidic through alkaline.
Temperature
This is a cold-hardy tree, comfortable roughly from -40°F to 104°F (-40°C to 40°C) across USDA zones 3 to 9. Winter poses no threat to a healthy ash.
Humidity
Outdoor ambient humidity does not matter to ash. Air movement through the canopy matters more, since stagnant, damp air keeps foliage wet and invites leaf fungus such as anthracnose.
Feeding
Established landscape ash rarely needs feeding. Apply a balanced complete fertilizer in spring only when a soil test shows a real deficiency, not on a calendar.
Pruning
Prune in late winter while the tree is dormant, removing dead, broken, and crossing limbs and shaping 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 at 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 the broad crown room.
When to plant
Plant balled-and-burlapped or container stock in spring or fall whenever the soil is workable, which leaves a wide window. The small flowers open in April and May, just before or as the leaves emerge, and the winged seeds ripen through 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
A single pest, the emerald ash borer, outweighs every other problem on this tree, so learn its signs first. The rest are largely cosmetic and tend to resolve on their own. Match the symptom below, then decide whether to act or simply keep watch.
Crown dying back from the top down, with D-shaped holes in the bark
Catching this early decides the outcome, so read the signs carefully. The cause is the emerald ash borer, an invasive beetle whose larvae feed in winding tunnels just under the bark and girdle the tree from the inside, severing the vascular tissue that moves water and sugars; nearly every untreated ash eventually dies from it. Look for tiny D-shaped exit holes, the winding S-shaped galleries packed beneath bark that has begun to loosen, pale woodpecker flecking where birds have stripped bark to reach larvae, and a crown that thins from the top down because the upper limbs lose their supply first. A high-value tree still holding more than half its canopy, meaning under 30 to 50 percent loss, can often be saved with a professional systemic insecticide; a heavily infested one should come down quickly before it turns brittle, and never move ash firewood, the single fastest way the beetle spreads.
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
Gradual thinning, dense witches-brooms, and small chlorotic foliage
This is ash yellows, a phytoplasma infection that colonizes the tree's phloem and thins it gradually over several years, throwing up tufted witches-brooms of weak, crowded growth as the plant struggles to compensate. There is no cure once it is established. Steady watering and a good mulch layer keep the tree as vigorous as possible and slow the slide, and once declining limbs grow brittle enough to threaten what is below, that is the point to have the tree removed.
Fix:
- No effective cure exists
- Maintain vigor with proper watering and mulch
- Remove severely declining trees that become hazardous
Vein-tracing brown patches on foliage following cool, damp weather
This looks worse than it is. The cause is ash anthracnose, a fungus that flares in cool, damp springs because prolonged leaf wetness lets the spores infect new tissue, leaving irregular brown patches that trace the veins and margins. It concentrates in the lower, inner canopy where airflow is poorest and leaves stay wet longest, and it can force an early leaf drop, yet a vigorous tree simply re-leafs and carries on. Rake and destroy the fallen leaves to cut the spore load that overwinters, thin the canopy to move air through it, and skip the fungicide on an established tree, which does not need it.
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
Knotted galls, greening then browning, massed on spring blooms
This one calls for no action at all. The lumps are ash flower galls, produced when a tiny eriophyid mite feeds on and distorts only the male flower clusters; the tree's hormones overgrow the tissue into knotted clumps in response. The galls ripen to brown and hang on as dry masses, but they never touch the leaves or affect the tree's health in any way, so there is nothing to treat. Rake up the fallen galls only if the look bothers you.
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 for cats, dogs, and people, with no known poisoning risk to grazing animals or birds. The one thing to keep straight is that it is not the same as the unrelated mountain ash or prickly ash.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats None
True ash is absent from the ASPCA toxic list, so a cat batting around a stray leaf is no cause for concern. A large mouthful of any plant material can still unsettle a sensitive stomach, which is true of greenery generally rather than ash in particular.
Dogs None
Ash is non-toxic to dogs, and a chewed twig or leaf carries no poison. The real hazard is a splinter from gnawed wood, not anything in the plant.
Horses None
Horses graze around ash with no trouble, since the tree contains no toxin known to affect them.
Rabbits None
A rabbit nibbling ash leaves or bark is safe. Introduce any new green slowly so the gut has time to adjust, as you would with any change in diet.
Livestock None
Cattle, goats, and sheep can browse ash foliage without risk of poisoning; in some regions the leaves have long been cut and stored as fodder.
Birds None
Backyard and wild birds are unharmed by ash, and many feed on the winged samaras through fall and winter.
People None
Ash poses no poisoning risk to people, and the wood has been worked and burned safely for generations. It is not grown as a food crop, so there is nothing on it meant for 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, selected to skip the samara litter and to hold a reliable shape. These are the names worth knowing.
Patmore (green ash)
A seedless male green ash hardy to zone 2, holding an even, tidy form and turning a clear yellow in fall.
Marshall's Seedless (green ash)
A seedless male green ash popular for decades, valued for glossy, dark green summer foliage and the absence of messy seed.
Autumn Purple (white ash, Fraxinus americana)
A seedless male white ash clone whose fall color runs reddish purple to deep mahogany and lingers on the tree.
How to propagate
Ash grows readily from seed if you have the patience for the long chill it needs, but 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 grows readily from seed if you can wait out the chill it requires, because the seed sits dormant and will not germinate without a prolonged cold period to break that dormancy. Gather the winged samaras in fall, then hold them in cold, moist stratification near 35°F to 40°F (2°C to 4°C) for about 90 to 120 days before sowing for spring germination, and grow the seedling on for one to two seasons until it is ready to transplant. Named seedless cultivars do not come true this way, so they are budded or grafted instead.
GraftingDemanding
Late summer budding or dormant-season grafting · 1 to 2 years to a saleable tree
This is nursery work rather than a 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 is what keeps them seedless and true to type. Figure one to two years for the graft to grow into a saleable tree.
Myths
Ash and mountain ash are the same tree.
The two share a common name and nothing else. True ash is Fraxinus in the olive family, identifiable by opposite, pinnately compound leaves, whereas mountain ash is Sorbus in the rose family, with alternate leaves and clusters of red berries. They sit in entirely separate genera, so care guidance written for one rarely transfers to the other.
A crown that is thinning out will recover on its own once the tree gets more water and feeding.
Treat this claim with caution. Dieback that starts at the top, paired with D-shaped exit holes and S-shaped galleries under the bark, points to emerald ash borer rather than thirst or hunger. No amount of watering or feeding halts the larvae girdling the trunk, and untreated trees typically die within about 2 to 5 years. Only early professional systemic treatment gives a valued tree a genuine chance.