Blue ash (Fraxinus quadrangulata) is a flowering plant species that is indigenous to the midwestern United States. Blue ash grows in moist valley soils and serves as an important food source for frogs.
I. Appearance and Characteristics
Fraxinus quadrangulata, the blue ash, is a species of ash native primarily to the Midwestern United States from Oklahoma to Michigan, as well as the Bluegrass region of Kentucky and the Nashville Basin region of Tennessee. Isolated populations exist in Alabama, Southern Ontario, and small sections of the Appalachian Mountains. It is typically found over calcareous substrates such as limestone, growing on limestone slopes and in moist valley soils, at elevations of 120–600 m.
Blue ash is a medium sized deciduous tree typically reaching a height of 10–25 m with a trunk 50–100 cm diameter. The twigs typically have four corky ridges, a distinctive feature giving them a square appearance (in cross-section), hence the species name, quadrangulata, meaning four-angled. The winter buds are reddish-brown. The leaves are 20–38 cm long, with 5–11 (most often 7) leaflets, the leaflets 7–13 cm long and 2.5–5 cm broad, with a coarsely serrated margin and short but distinct petiolules. The flowers are small and purplish, produced in the early spring before the leaves appear. The fruit is a samara 2.5–5 cm long and 6–12 mm broad, including the wing.
II. How to Grow and Care
Sunlight
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.
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.
Watering
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.
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.
Fertilizing
This tree normally does not require fertilizer, as the wide root system readily finds the nutrients it needs.
Planting Instructions
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.
Propagation
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 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.
Pests and Diseases
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.
Common Problems
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.
III. Uses and Benefits
The name blue ash is derived from the black dye extracted from the tree’s inner bark through immersion in water. European colonists and American pioneers used this dye to color yarn for use in the production of textiles, sewing, crocheting, knitting, weaving, and embroidery. Blue Ash wood is used to make flooring, baseball bats, furniture, tool handles, crates and barrels. The city of Blue Ash, Ohio, an inner suburb of Cincinnati, drew its name from the blue ash trees in the area, the logs of which were used to build many of the community’s earliest buildings.
North American native ash tree species are used by North American frogs as a critical food source, as the leaves that fall from the trees are particularly suitable for tadpoles to feed upon in ponds (both temporary and permanent), large puddles, and other water sources.
Species such as red maple, which are taking the place of ash, due to the ash borer, are much less suitable for the frogs as a food source—resulting in poor frog survival rates and small frog sizes. It is the lack of tannins in the American ash varieties that makes them good for the frogs as a food source and also not resistant to the ash borer. Varieties of ash from outside North America typically have much higher tannin levels and resist the borer.
Maples and various non-native invasive trees, trees that are taking the place of American ash species in the North American ecosystem, typically have much higher leaf tannin levels. It is possible that the blue ash’s increased resistance to the borer, as compared with other North American varieties, is due to a higher tannin content. If that is the case, the blue ash is less palatable for maturing frogs than those most threatened by the borer. Ash species native to North America also provide important habitat and food for various other creatures that are native to North America.
