How to Grow and Care for Blue Ash

If you live in the Midwest, from Kentucky to Oklahoma, you most likely have seen the blue ash tree growing along the slopes of a limestone cliff or in the moist bottomland of a fertile valley. The blue ash is a medium-sized tree that can be identified by its squarish twigs, which is the origin of the botanical name, Fraxinus quadrangulata, or four-cornered ash. The size of the blue ash, 50 to 75 feet, makes it a great shade tree.

But blue ash trees are getting harder to find. Sadly, the tree is now declared endangered due to the emerald ash borer (EAB), which plagues all ash trees in North America. Although blue ash is considered somewhat more resistant to EAB (by some estimates, about 60 to 70 percent of infected blue ash trees survive), ash trees, in general, are no longer readily available for sale in many regions.

Should you be able to purchase one and are willing to have preventive treatments done regularly, blue ash is usually planted as a potted nursery plant in the fall or spring. While still a faster grower than many deciduous hardwood trees, blue ash has a slower growth rate than the other ashes. It will add 8 to 12 inches per year in most circumstances.

Blue Ash Care

Growing comes easy to blue ash trees and historically this species has been a good choice for areas with alkaline soils where many deciduous trees struggle, such as areas with limestone bedrock. The onslaught of emerald ash borer, however, has changed the official thinking about all ash trees, including the blue ash. If you are able to find blue ash for sale, be prepared for the expense of preventive chemical treatments to protect the tree against EAB—which has become a must for anyone planning to grow an ash tree.

Whether planting as a container sapling, a bare-root specimen, or a balled and burlapped tree, remember that blue ash will get fairly large and needs plenty of space around it—at least 30 to 40 feet. But provided you can avoid EAB, blue ash needs little more than regular watering in order to thrive.

Light

Blue ash does best in full sun. Look around the area in which you intend to plant the tree and consider the growth rate of surrounding trees and the shade they may cast over your planting site. Also, consider the shade your ash tree will cast on the rest of your landscape as it grows.

Soil

The tree prefers a moist, slightly sandy loam that drains well. However, of all ash trees, this species is the most tolerant of dry soil. Blue ash tolerates just about any soil pH, but it has been a favorite choice for areas with alkaline soils where many deciduous trees struggle.

Water

Water your ash tree frequently when the tree is young, about 1 to 2 inches per week. As it matures, unless you are in a dry area or experiencing especially dry weather, watering should not be necessary.

Temperature and Humidity

Hardy in zones 4 to 7, blue ash trees can handle high temperatures as well as high humidity. They thrive on warm, humid midwest summers, and easily survive winter temps down to minus 20 degrees Fahrenheit.

Fertilizer

This tree normally does not require fertilizer, as the wide root system readily finds the nutrients it needs.

Types of Blue Ash

Where it is available, blue ash is often sold as the pure species form, Fraxinus quadrangulata. However, there is a popular cultivar, 'True Blue' that is faster growing and especially good for alkaline soils. It is known to keep its dark green color longer than the species and does not "yellow out" in the way that the species tree does.

Pruning

The blue ash grows moderately fast (though slower than other ash species), so pruning is recommended two to three years after planting. Pruning after the leaves drop in the winter, when the tree is dormant, is optimal. Using proper tools that have been sanitized is very important, as well.

When pruning an ash tree, your main goal is to establish a central leader, or main trunk, and to remove any inner branches. Be sure to remove any low-hanging, dead, or dying branches. Pruning to remove broken or diseased limbs should be done whenever you notice them.

Take note if your tree seems to have an abundance of dieback, as this can signal a major issue.

Propagating the Blue Ash

The blue ash is notoriously hard to propagate from cuttings, which is one of the reasons this species can be hard to find in nurseries. Trees offered in the nursery trade are usually grown from seed, and that can be a long process.

How to Grow Blue Ash From Seed

To start a blue ash from seed, you will need to stratify the seeds by first removing a portion of the hard shell by soaking the seed in water for 24 hours. Then you will need to warm stratify, which means to simulate the seasonal temperature changes, for 60 days. Placing the seeds in a sunny window is often sufficient. Follow this by cold stratifying for 60 more days. Storing the seeds in a refrigerator is one way to do this; leaving the seeds outdoors in cold winter weather is another method.

At this point, you can sow your seeds at a depth of 3/8 inch in a good-quality seed starting mix. Keep the potting mix moist and set the container in a warm, bright location until the seeds germinate and sprout. When they reach several inches in height, the seedlings can be transplanted into larger pots filled with standard potting mix and grown on until they are large enough to plant in the landscape.

Overwintering

Established trees don't need any help with surviving the winter. Smaller trees might benefit from a healthy layer of mulch over the roots during the first few years.

Young ash trees have relatively thin bark that can benefit from being shielded by hardware fabric or another impenetrable shield to prevent gnawing by rabbits and other creatures. After two or three years, the bark is sufficiently thickened and this protection is no longer necessary.

Common Pests & Plant Diseases

The blue ash is susceptible to the emerald ash borer (EAB) but it has a somewhat higher survival rate than other ashes, possibly due to higher tannin levels that may make it a less appealing target. Still, few experts recommend planting any species of ash tree as landscape specimens at this time. Preventive treatments with emamectin benzoate have proven to have some success at helping established trees resist infestation, but this treatment needs to be repeated every other year, at considerable cost, to be reliably effective. Only plant the blue ash if you're willing to take on the cost of prevention or treatment if it does occur.

Lilac borer is another common insect pest to plague the blue ash. The symptoms are much like those of the emerald ash borer: dieback, random leafy growth, and of course, round boreholes. To treat lilac borer, the standard pesticide applications traditionally used have been permethrin or bifenthrin, which are available at most garden centers.

Ash yellows is the most serious plant disease you are likely to encounter. It damages the tree’s vascular system. The symptoms to look for are slow twig growth and rapid dieback, which is why pruning and inspecting your ash regularly is so important. There is no known cure for ash yellows and it is recommended you remove the tree as soon as the disease has affected more than half the tree. 

Anthracnose is a fungal disease that leaves brown spots on leaves and is often confused with frost damage. It leads to twig death, leaf loss, and eventual dieback. To treat this, try a copper-based fungicide.

How to Get Blue Ash to Bloom

A mature blue ash produces tiny purple flowers in spring, emerging at the same time as the leaves sprout. Although not unattractive, these flowers are not particularly showy, either, and there's no reason to lament a tree that does not flower. In fact, lack of flowers will prevent the tree from producing the seed clusters that can be quite messy when they are shed in the fall.

Common Problems With Blue Ash

A blue ash that manages to avoid EAB and other pest and disease issues is usually a fairly trouble-free shade tree. But like most ashes, blue ash has relatively brittle wood that makes it susceptible to wind damage. It's best to plant this tree well away from buildings and to prune out broken limbs as soon as possible to reduce avenues for borers and fungi to infiltrate. This brittleness gets more pronounced on older trees.

And like all ashes, blue ash can be a messy tree in the autumn as the ripened seed clusters are falling. Lawn droppings can be mowed up with a bagging mower, but this is not a tree you want overhanging a patio, deck, or driveway.