Tree of Heaven: April’s Weed of the Month
Tree of Heaven (Ailanthus altissima) sounds lovely – but it is not. This very invasive deciduous tree from China has spread vigorously along waterways, roadways and agricultural areas. Tree of Heaven is also a host for the invasive brown marmorated stink bug. It has pale grey bark, light brown twigs and large, pinnately compound alternate leaves. Each compound leaf is made up of 11-25 leaflets, arranged opposite each other. The compound leaves can grow up to four feet long, and each leaflet has one to several glands near their base.
Identification can be difficult as its leaves are similar to black walnut, sumac and ash. Tree of Heaven can be distinguished by its fuzzy brown twigs and seed heads that stand erect. It also has a strong stench, particularly from its flowers, that has been likened to cat urine. Perhaps the best identifiers are the glands at the base of the leaves, as seen here.
Tree of Heaven is a prolific seed-producer which is why it is out-competing native species. It has large showy clusters of small yellow-green flowers in June that produce flat, single-seeded winged fruits in summer. One tree can produce an estimated 325,000 seeds each season. It can thrive in harsh conditions, which is why you see it growing up in parking strips and through riprap.
Tree of Heaven can be controlled numerous ways. The best way is to not plant it in the first place. The next easiest is to pull out the small seedlings before the tap root develops. Larger trees can be cut, although that will only slow it down, not kill it. Cutting will also cause the tree to re-sprout vigorously. If cutting down trees, it is recommended that cutting be followed by painting the stump with an herbicide to improve control. Check with the OSU Extension Master Gardeners for advice on effective herbicides and always follow the herbicide label instructions. Subsequent herbicide treatment of root suckers may also be needed.
You can also reduce the spread of this tree by cutting and bagging the seed heads on female trees. Tree of Heaven is dioecious, meaning plants are either male or female. Targeting reduction of female trees is a strategic way to slow this weedy and persistent invasive.