How to Kill a Yucca

Q. I had a yucca plant that has been in the ground 15 years, so you can guess how large it was. It was encroaching on the sidewalk, so felt we had to remove it.  We were unable to dig it up, so we took a chain saw and cut if off at the base and turned off the water. Within 3 days, we had 20 new shoots. Can you advise how to kill this plant?

Joshua tree (Yucca brevifolia)  in Sandy Valley. A xeric (watered infrequently) yucca.  Little rain in the Mojave Desert causes a small amount of growth every year.

A. I dont know what kind of yucca this is. There are yucca that are "mesic" and others that are "xeric". In the Mojave Desert climate it will eventually die because of a lack of water unless something is growing a few feet away that needs water. If water from other plants is close enough, the roots will take that water and use it. 

Another type of Yucca (probably Y. gloriosa) which is mesic (watered more often than xeric) and more commonly found planted in the Southeastern US.

When you cut it with a chainsaw you leave behind a small amount of stem attached to the roots. The stem, and not the roots, can provide new growth at certain times of their lives or when they are damaged. In most cases I know of, the roots alone cannot do this. Only if there is some stem (trunk) tissue left behind.

There is a portion of the stem just above the roots which can send rhizomes into the surrounding soil to produce new plants called "pups". Not all yuccas will do this. Evidently, yours is one that does. Some Joshua trees can produce pups while others will not. Conservationists believe it is a trait of Joshua's growing at higher elevations but not lower elevations. Regardless, if you can kill all of the stem tissue down to the roots this should stop any regrowth of the plant. You can try drilling some holes in the remaining stem about half an inch from its perimeter and as deep as you can. Fill these holes with a diluted weedkiller or use copper sulfate. If it's a liquid, fill the holes several times. If it's a solid, fill the holes once. This should kill the stem and prevent any new growth.