How to Grow and Care for Green Ash

Historically, green ash (Fraxinus pennsylvanica) has been an incredibly hardy, fast-growing, and adaptable deciduous tree that could thrive in almost any habitat in its native North America. An upright tree with a rounded but irregular canopy, green ash has compound leaves with five to nine leaflets. The fall foliage is yellow.

But although once among the most popular shade trees for public and private properties, green ash trees are now deeply threatened by the devastating and fast-spreading emerald ash borer (EAB). EAB is treatable, but it's quite costly. As a result, purposefully planting new green ash trees is not recommended and is even banned in some regions. Check your state's regulations and think carefully before you introduce a new green ash tree to your area—it may end up being subject to removal and could spread the disease to trees situated nearby. If you are able to purchase and plant an ash tree, you should be prepared for the repeated expense of professional chemical treatments to prevent EAB infestation.

Where allowable, green ash trees are normally planted as potted nursery trees in the spring. They are relatively fast-growing trees, adding at least 2 feet per year and growing to as much as 25 feet in a single decade.

Green Ash Care

Green ash prefers well-drained, moist soils and a full sun position, but they can grow quite well in a wide range of conditions—which is perhaps the reason they became so popular (and overplanted, by some estimates). This popularity has turned out to be the tree's weakness: Because so many ash trees filled urban forests over the past decades, the emerald ash borer has been able to devastate North American tree populations in short order.

If you are lucky enough to have a healthy green ash tree, though, you will recognize its ability to tolerate just about any soil type, any weather condition, both drought and wet conditions, and urban pollution.

Light

Green ash prefers to be positioned in a location where they get plenty of sun. They will not do well where shaded by surrounding trees.

Soil

These trees thrive in moist, fertile soil conditions. They aren't particular when it comes to pH levels, and good growth is possible in loamy, sandy, or clay soil types.

Water

Green ash trees prefer to be kept consistently moist, but they also copes well during short periods of drought. Green ash also tolerates short submersion in water, such as during seasonal flood cycles.

Temperature and Humidity

Given their widespread use across the United States, it's no surprise that green ash trees can cope with a wide variance in temperatures. They're known for being impressively cold-hardy and will thrive in zones 3 to 9 if they can be protected against ash borers.

Fertilizer

Young trees benefit from a slow-release fertilizer with decent phosphorous levels. If you have a mature tree already established in your garden, it won't need fertilizing unless it's showing signs of stress.

Types of Green Ash

There are several popular cultivars of green ash, though unfortunately none yet with demonstrated resistance to emerald ash borer.

• ‘Marshall's Seedless' has been the most commonly used green ash cultivar for decades. It is a seedless, fast-growing variety with dark green glossy leaves and yellow fall color.
• ‘Patmore' is a newer cultivar derived from ‘Marshall's Seedless.' It is slower growing and has a more uniform shape.
• ‘Summit' has an upright growth habit with leaves that are lighter and less glossy than 'Marshall's Seedless'. It has good yellow color in the fall.
• 'Cimmaron' is known for its striking red and orange foliage in the fall. It is a seedless cultivar with branches that grow more laterally than other cultivars.

Pruning

Young green ash are sometimes pruned to help manage their form. These trees look their best when shaped to encourage a central leader with major branches alternating along the trunk. With young trees, low crotches should be eliminated by pruning away the lesser of the two adjoining branches. Pruning is best done during winter dormancy, though diseased or broken limbs should be promptly removed whenever you notice them.

Mature green ash trees won't need much pruning, except to remove branches that become diseased or broken. Major winter pruning may become necessary if the tree becomes too wide or tall for its space.

Propagating Green Ash

Although green ash trees can be easily propagating through cuttings, this isn't currently recommended given the problems with emerald ash borer.

How to Grow Green Ash From Seed

Propagating a green ash tree from seeds is a time-consuming, somewhat difficult task, as the seeds require repeated rounds of cold stratification over a period of more than a year. Not all seeds are fertile, so it can take many attempts to successfully sprout a seedling. The dried seeds should be planted about 1/2 inch deep in a pot filled with moist potting soil, then sealed in a plastic bag and chilled for one to three months.

Take the pot out and place it in a warm (70- to 75-degree Fahrenheit) location for three months. Place the pots in plastic and chill again for one to three months. Repeat this process until seeds germinate and sprout. The entire process can take as much as two years before you have viable seedlings to plant outside.

But any form of propagation is questionable at this time, given this tree's current susceptibility to emerald ash borer.

Common Pests and Plant Diseases

Green ash trees are susceptible to quite a large number of insect and disease problems, but these are all dwarfed by the very serious problem of emerald ash borer.

Emerald ash borer (EAB) has been present in the United States since at least 2002 when it was brought over from Asia—probably in wooden shipping pallets. This devastating infestation is caused by the larvae of the Agrilus planipennis beetle. They feed on the tree from the inside and prevent it from being able to carry nutrients and moisture effectively.

It's thought that adult beetles can fly distances of up to 15 miles and the larvae can stay inside timber cuttings, so the spread of the pest has been fast-moving and difficult to eradicate. If you suspect an existing tree has an EAB infestation, quickly consult a professional (a certified arborist).

EAB is treatable, though treatment is quite expensive. If EAB goes untreated, an infestation will kill a tree within a few years.

Preventive treatments with emamectin benzoate have proven to have some success in helping trees resist infestation. These preventive treatments need to be repeated every two years, however, and they need to be applied by licensed professionals. Maintaining a healthy ash tree is an expensive proposition.

Other problems you may face with green ash include carpenter worm, oyster shell scale, leaf miners, fall webworms, ash sawflies, and ash leaf curl aphid. Possible disease problems include fungal leaf spots, powdery mildew, rust, anthracnose, cankers, and ash yellows.

Common Problems With Green Ash

The Tree Is Dying From the Top

Unfortunately, an ash tree that begins to die back beginning at the upper crown may very well already be in the throes of emerald ash borer infestation. You should have the tree inspected by a professional arborist.

Limbs Are Breaking Off

This tree species is notoriously susceptible to wind damage, so if you live in an area with frequent thunderstorms or other high winds, you should expect to do cleanup regularly. Ash is a brittle wood, and the limbs will snap without much provocation.

A large, old tree is especially susceptible to wind damage, and you may even want to consider proactively having it removed to avoid the potential catastrophe of having it fall on your home.

Tree Is Very Messy

Many ash trees produce an enormous volume of seeds, and you can expect to do a lot of raking at certain times of the year. This is one reason why seedless cultivars are very popular.