Evan X. Merz

Tree of Heaven is one of the most dangerous and difficult to control weeds in the state of California. If you see it in your yard or in your public parks, you should take action immediately to kill and remove it.

Unfortunately, identifying Tree of Heaven can be kind of difficult. For years I thought California was overrun with Tree of Heaven, but really it's just difficult to distinguish California's native Black Walnuts from Tree of Heaven at a distance. The only way I know of to identify Tree of Heaven with certainty is by examining the compound leaves very closely. The leaflets have a distinctive notch at the base near the petiole that doesn't exist on native species with similar compound leaves.

Controlling Tree of Heaven

Unfortunately, controlling Tree of Heaven is extremely difficult. I suggest physical removal of the tree and the long roots as soon as possible while the trees are young. More mature trees can only be removed using a complex procedure that has be executed at exactly the right time of year. Here's the process as described by UCANR.

Careful treatment with an herbicide in July, August or September, followed by removal of the trees about 30 days later offers the greatest likelihood of success. According to CDFA, herbicides containing the active ingredients glyphosate or triclopyr are effective and pose minimal risk to nontarget plants.

Spray all the trees' leaves if tree height allows for safe application with a backpack or hand pump sprayer. For dense or extensive infestations and tall trees, treat small, low growth with a sprayer, then follow up with hack-and-squirt application on larger stems. Hack-and-squirt introduces the herbicide into the stem using spaced cuts below the last live branch. With a hatchet or similar device, make downward-angled incisions, evenly spaced around the stem. The cuts must penetrate through the bark into the living tissue or sapwood (the outer area of lighter-colored wood in the stem cross-section). Leaving uncut living tissue between the hacks will allow the herbicide to move to the roots. Using a low-volume backpack sprayer or spray bottle, fill the cuts with a concentrated mixture of the herbicide.

Thirty days later, cut down the trees. Follow up by monitoring the area for regrowth and repeating the treatment protocol as needed.

If it's not August or September, then you can try removing by hand, but you need to be very thorough if you want to remove it all.

For even more of my gardening content, check out Evan's California Garden on Facebook.