Blue Ash Tree Profile

Close up of blue ash leaflets

Courtesy of Purdue University 

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 bottom land of a fertile valley. The blue ash is 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 "Blue" in the blue ash comes from a substance found under the bark that, when exposed to air and immersed in water, was used to make a fabric dye by early Americans.

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, which plagues all ash trees. Being one of the rarer trees in the genus Fraxinus, it has relatively high EAB resistance compared to other ashes and much higher survivability rates compared to the white ash. Even though it is resistant to this devastating pest the tree still hasn't caught on in the nursery trade and might be hard to find.

Botanical Name Fraxinus quadrangulata
Common Name Blue Ash
Plant Type Tree 
Mature Size 50.00 to 75.00 feet
Sun Exposure Full Sun
Soil Type average, dry, well-drained 
Soil pH  6.8-7.2
Bloom Time April to May
Flower Color Purple
Hardiness Zone 4 to 7
Native Area Midwestern United States

How to Grow a Blue Ash Tree

Growing comes easy to blue ash trees, though the maintenance can be tricky. If you are lucky enough to find a blue ash tree and are looking to start from a sapling or balled and burlapped, the most important things to remember are that your tree will get large so plant it with plenty of room to grow and you should plan now for the future so it won't cause your property issues down the line. It will need full sun, and it prefers alkaline soil , which means a soil pH greater than seven. If given those conditions, and proper watering your tree will do beautifully, if not bothered by pests or disease, which, eventually, is the downfall of most ash trees.

Close up of a blue ash bloom and stem illustrating its square shape
Courtesy of Purdue University  

Light

Plant your blue ash in full sun.

Soil

The tree Prefers moist, humus-y loams mixed with sand but, of all ash trees, it can tolerate dry soil the most. Ideally you would plant a blue ash in alkaline soil. You can test your soil with a kit or with this nifty DIY method. This tree is native to the Midwest which is rich in limestone and has a naturally high pH. If your soil is acidic you can lime out your soil to achieve the desired pH, 6.8-7.2.

Water

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

Fertilizer

This tree normally does not require fertilizer.

Propagating the Blue Ash

The blue ash is notoriously hard to propagate from cuttings, which is one of the reasons the blue ash is not often available. Trees that are sold are usually grown from seed which is a long process. To start your tree from seed you will need to scarify the seeds, or remove a portion of the hard shell on the seed, 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. Follow by cold stratifying for 60 more. At this point you can sow your seeds at a depth of 3/8th of an inch into a good quality seed starting mix.

Pruning

Pruning your ash tree is going to be the most important maintenance you will be performing. The blue ash grows moderately fast 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, your main goal is to train to establish a central leader, or main trunk, and to remove any inner branches. Also be sure to remove any low hanging branches While pruning, be sure to look for and remove any dead or dying branches. Take note if your tree seems to have an abundance of dieback that could signal a major issue.

The most important thing to remember with Ash tree waste is to always check to see if you area is are under quarantine before you dispose of anything you prune. You can call your local DEP or extension agency for clarification.

Emerald Ash Borer

The blue ash is still susceptible to EAB but luckily it seems to have a high resistance. The experts believe this is due to the higher tannin levels in this particular species of ash, which makes the EAB prefer to terrorize other ash trees.

Other Pests and Diseases

Ash Yellows is the most serious disease you are likely to encounter. It damages the tree’s vascular system. The symptoms to look for are slow twig growth and rapid die back which is why pruning and inspecting your ash is so important. There is no known cure for Ash Yellows and it is recommended you remove the tree as soon as Ash Yellows 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 deaf, and leaf loss and eventual dieback. To treat this, try a copper-based fungicide or a bordeaux mixture.

Lilac borer is the most common insect pest to plague the blue ash. The symptoms are much like that of the emerald ash borer -dieback, random leafy growth, and of course bore holes. The main difference being that the bore holes are round not D shaped. To treat lilac borer the standard pesticide applications traditionally used have been permethrin or bifenthrin, which are available at most garden centers.

Warning

Always read and carefully follow all precautions and directions provided on the container label. Store all chemicals in the original labeled containers away from food, and out of the reach of children, and animals!