Dear Master Gardeners,
I manage some “garden zones” on my apartment buildings property; the particular garden zone of concern is a giant concrete planter that a “weed tree” has unfortunately infiltrated. It seems to have grown an underground network. I have learned in recent years that if I cut down 1 baby tree down that another 3 might come up in other areas of the container; I don’t necessarily discover them right away as they may spring up under the cover of existing shrubs. At this point, I am afraid to cut down the baby weed tree (pictured) & a couple of others just like it that are in the same container. Can u please tell me when is it appropriate to use the girdling technique on these invasive, bad smelling trees & when is it appropriate to just dig them out & cut them down. The one pictured here sprang up just this season. I can actually see part of the underground connection between a couple of them; the rest of the connection may be to difficult to get to.
Thank you for contacting Toronto Master Gardeners.
From your picture and from your description of the tree being foul smelling, I believe you have a Tree of Heaven, Ailanthus altissima. It was introduced from Aisa to North America before the 1850’s. This is an invasive tree here in Ontario and south of the border throughout the states. They grow fast and are stubborn as you have seen.
This tree is very difficult to eradicate. Cutting and girdling lead to an increased number of suckers. When pulling out roots any pieces left behind will sprout into new shoots. Cutting back alone will not eradicate this tree. The leaves can be an irritant so you should wear gloves when handling this plant. Any pieces removed from the plant should go in the garbage not in garden waste.
The only way to fully remove the tree is with a combination of techniques. The use of herbicides is necessary. It needs to be applied during seasons with maximum growth. You will need to cut into the wood to apply the herbicide and you will need to do it repeatedly. It can be a bit labour intensive but will ultimately work. You will need to visit your local nursery to see which herbicides are available and which will work on the Tree of Heaven. Along with this if you keep cutting back the growth you will get suckers but it will ultimately help weaken the plant. Shade can also help weaken the new shoots. A good thick layer of wood chips may also slow down shoot growth.
I have included a link below that has more information on the plant and how to eradicate it.