Tree · Oleaceae
Green Ash
Fraxinus pennsylvanica
A tough, fast-growing shade tree for tricky ground, 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 quick-growing native shade tree in the olive family, and it thrives on ground that defeats fussier trees, so it is a forgiving choice. Flooding, drought, wind, even alkaline soil barely faze it, which is exactly why it once lined countless streets and parking lots across North America. Its opposite, pinnately compound leaves and winged seeds, the ones called samaras, are how you tell it apart from the lookalike mountain ash, a rose-family tree it is not actually related to.
The tree pays you back with lovely form and color. It reaches 50 to 70 feet (15 to 21 m) tall with a broad, even crown, fills out fast in its early years, and turns a clean yellow each autumn. Cold simply does not scare it, since it shrugs off winters down to -40°F (-40°C) and feels at home across USDA zones 3 to 9.
There is one honest caveat to share, and knowing it makes you a better steward. The emerald ash borer, an invasive beetle, has killed hundreds of millions of ash and will eventually find almost any untreated tree in its way. Most extension services no longer suggest planting new green ash, so the wise and caring approach is to tend the trees already standing and keep a close eye on them.
Care
The short version: give it full sun and plenty of elbow room, keep young trees evenly moist, and prune in late winter to build a sturdy frame. A settled-in ash asks almost nothing of you, though it does deserve a watchful eye for borer.
Light
Set ash in full sun for the fullest crown and the best autumn color, though it will happily accept partial shade if that is what you have. The more direct light it soaks up, the stronger and more even the tree becomes.
Water
Keep young trees in steadily moist, humusy loam while their roots get settled, offering a drink roughly every 1 to 2 weeks during dry spells. Once it is established, ash takes occasional flooding and drought in stride and will rarely call on you for help.
Soil
Few trees are this easygoing about their footing. Ash will settle into clay, loam, or sand as long as it drains reasonably well, and it is comfortable across a wide pH from 5.5 to 8.0, acidic right through to alkaline.
Temperature
This is a deeply cold-hardy tree, content anywhere from about -40°F to 104°F (-40°C to 40°C) and right at home in USDA zones 3 to 9. A healthy ash has nothing to fear from winter.
Humidity
Whatever the outdoor humidity happens to be, ash takes it as it comes, so there is nothing to fuss over. What matters more is air movement through the canopy, since still, damp air is what tempts leaf fungus like anthracnose to set in.
Feeding
A settled-in landscape ash hardly ever needs feeding. Offer a balanced complete fertilizer in spring only when a soil test reveals a genuine shortfall, never just because the calendar says so.
Pruning
Prune in late winter while the tree rests, clearing out dead, broken, or crossing limbs and shaping a sound structure early on. The wood runs brittle and is prone to wind and ice damage, so the good form you set today pays off for decades.
Repotting
none
Size & spacing
Green ash grows into a sizable tree at 50 to 70 feet (15 to 21 m) tall and spreads 35 to 50 feet (11 to 15 m) across. Plant it well away from buildings, walkways, and power lines, and let that broad crown have all the space it wants.
When to plant
You will plant ash as balled-and-burlapped or container stock in spring or fall, anytime the soil is workable, so there is a generous window to work with. The little flowers open in April and May, right before or just as the leaves unfurl, 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 stands above everything else on this tree, so it is the first thing worth learning to recognize. Happily, most of the other troubles are merely cosmetic and clear up on their own. Find the picture below that matches what you are seeing, then decide whether to step in or simply keep watch.
Top of the crown thinning out, with small D-shaped holes punched through the bark
Take a breath, because spotting this early is half the battle. What you are seeing is the emerald ash borer, an invasive beetle whose larvae quietly girdle the tree from beneath the bark, and sadly it claims nearly every ash left untreated. The tells are little D-shaped exit holes, the curling S-shaped galleries hiding under bark that has gone loose, and the pale flecking left where woodpeckers have worked, all while the crown fades from the top down. There is genuine hope for a prized tree showing less than 30 to 50 percent canopy loss, since a professional systemic treatment can pull it through; a tree already heavily infested is safest taken down quickly, and please never move ash firewood, which is how the beetle travels.
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
A gradual fade, with tufted witches-brooms and small, pale leaves
This is ash yellows, a phytoplasma infection that slowly thins the tree across several years and sends up tufted witches-brooms of weak, crowded twigs. There is, regrettably, no cure once it settles in, but you are not powerless. Keeping the tree well watered and mulched helps slow the decline and buys time, and when failing limbs start to pose a danger overhead, that is the moment to have the tree removed.
Fix:
- No effective cure exists
- Maintain vigor with proper watering and mulch
- Remove severely declining trees that become hazardous
Brown patches tracing the leaf veins once spring turns wet
This one looks alarming but is gentle on the tree, so you can relax. It is ash anthracnose, a fungus that wakes up during cool, soggy springs and paints uneven brown marks along the veins and edges of the leaves. You will usually see it lower down and toward the inside of the canopy, and it can knock leaves off early, yet a healthy tree recovers and pushes out fresh foliage on its own. Gather and toss the fallen leaves, open things up a little with pruning for better airflow, and there is no need to reach for fungicide on a mature 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
Lumpy green-turning-brown galls on the springtime blooms
Here is one you can simply let be. These are ash flower galls, the work of a tiny eriophyid mite that twists only the male flower clusters into odd little clumps. They ripen to brown and cling on as dry knots, but they never harm the leaves or the wider health of the tree, so there is nothing to fix. No treatment is needed, and if the look bothers you at all, raking up the fallen galls is purely your choice.
Fix:
- No treatment is needed because the galls are cosmetic
- Rake and discard fallen galls if desired
Toxicity
Here is some reassuring news. True ash does not appear on the ASPCA toxic-plant lists and is considered safe for cats, dogs, and people alike, 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, which are altogether different plants.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats None
You can let that worry go: true ash does not appear on the ASPCA toxic list, so a curious cat swatting at a stray leaf is perfectly fine. As always, a really big mouthful of any plant might unsettle a delicate tummy.
Dogs None
Ash counts as non-toxic for dogs, and a gnawed twig or leaf will not poison your pup. The thing to watch is a splinter from chewed wood, not the plant itself.
Horses None
Your horses can graze right alongside an ash with nothing to fear, as the tree holds no toxin known to trouble them.
Rabbits None
A rabbit that samples ash leaves or bark comes to no harm. Just ease any new green in slowly, the way you would with any fresh food, so the gut settles.
Livestock None
Cattle, goats, and sheep can browse ash foliage with no risk of poisoning, and in some places the leaves have long been cut as fodder for them.
Birds None
Backyard and wild birds are entirely safe around ash. Plenty of them happily feed on the winged samaras right through fall and into winter.
People None
There is no poisoning risk to people from ash, and the wood has been worked and burned safely for generations. It simply is not grown as a food, so nothing on the tree is meant for eating.
Keep ash firewood out of pet bedding only for splinter and mold reasons, not toxicity.
Varieties
Most ash sold for the home landscape are seedless male clones, picked so you avoid the litter of samaras and get a dependable shape year after year. These are the names you are most likely to come across.
Patmore (green ash)
A seedless male green ash that handles cold beautifully, hardy clear to zone 2, and keeps a neat, balanced shape with bright yellow color come fall.
Marshall's Seedless (green ash)
A favorite for many years, this seedless male green ash is loved for its glossy, dark green summer leaves and the welcome fact that it makes no messy seed.
Autumn Purple (white ash, Fraxinus americana)
A seedless male white ash clone grown for autumn color that shifts from reddish purple to deep mahogany and holds onto the tree nicely.
How to propagate
Ash takes to seed quite willingly if you have the patience for its long, slow chill, but the sought-after cultivars are another story and come off the grafting bench rather than out of a seed tray.
SeedModerate
Collect samaras in fall; sow after stratification for spring germination · 1 to 2 growing seasons to a transplantable seedling
Growing ash from seed is well within reach if you are happy to wait, since the seed is dormant and simply will not sprout without a good long chill. Collect the winged samaras in fall, then give them 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, letting the young plant grow on for one to two seasons before it is ready to transplant. Keep in mind the named seedless cultivars will not come true from seed, 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 firmly a nursery technique rather than a weekend backyard try, so do not feel you have to tackle it yourself. Named clones such as Patmore, Marshall's Seedless, and Autumn Purple are budded in late summer or grafted during the dormant season onto seedling rootstock, which is what 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.
It is an easy mix-up, but these two only share a name. True ash is Fraxinus, a member of the olive family, with opposite, pinnately compound leaves, while mountain ash is Sorbus, a rose-family tree carrying alternate leaves and clusters of red berries. Because they belong to entirely separate genera, care advice meant for one will rarely suit the other.
A thinning ash crown just wants a little water and feeding to perk back up.
This is one worth catching early. When the dieback begins at the very top and you turn up D-shaped exit holes and S-shaped galleries, the culprit is emerald ash borer, not thirst or hunger. No amount of watering or fertilizing will stop it, and untreated trees usually die within roughly 2 to 5 years. Only an early systemic treatment from a professional gives a cherished tree a real fighting chance.