Siberian elm seedlings can have a taproot that’s longer than the above-ground parts. Try to get all or most of the root when you pull them. Photo credit M. Thompson

Controlling Invasive Siberian Elm Trees

With Guest Author Dr. Leslie Beck

Question:

What’s the best way to kill elm seedlings?

- Mellene Pablo, Los Lunas

Answer:

I’m glad you’re asking this question when these weedy, invasive trees are small and relatively easy to control (emphasis on the “relatively”). We all know how precious shade is in New Mexico, and we love our trees for providing it, but weedy trees like tree of heaven, salt cedar, and the Siberian elm are real problems across the state.

This Siberian elm seedling was growing in one of my patio flower pots and in this photo is flanked by two Siberian elm trees growing in the alley. Photo credit M. Thompson

Several traits make Siberian elms (Ulmus pumila) one of the most despicable invasive tree species around. For one, they produce a ton of seeds each spring that fly around, sprout up everywhere, and unless you get them that first season, are very hard to pull. If they just germinated this year, they’re probably still small enough to pull by hand. Pull ‘em when you see ‘em. You’ll be sorry if you don’t.

Siberian elms also outcompete other, more desirable species, uproot walls around yards, don’t age well (branch breakage is common), and exacerbate allergies too. Albuquerque’s pollen ordinance bans these pests from being sold or planted within city limits. And did I mention the crazy amount of seeds they produce? In April, the seeds go whirling up and down my street in the wind, making cute pitter-patter sounds that make me shudder. Talk about nightmare on elm street.

Please note that not all elms are bad. Several elm tree species are highly recommended for our urban landscapes. These much more desirable trees include the lacebark elm (Ulmus parvifolia, aka Chinese elm), the American elm (Ulmus americana), and many well-liked cultivars and hybrids. The key is that these elms either drop seeds in the fall so the seeds don’t sprout everywhere in the spring or they’re sterile and don’t produce seeds at all.

I invited our NMSU Extension Weed Specialist Dr. Leslie Beck to help us figure out how to control these monsters in their different growth stages:

The most important thing to understand with invasive trees like Siberian elm is that in order to kill the tree so that it doesn’t just grow back from the roots, you have to kill the crown (the area where the trunk meets the roots). The crown can be damaged by grubbing, mulching, or burning the woody tissue, but if suckers have already rooted effectively into the soil (can’t physically remove them) you will still have to target their crown for effective control.

For mature trees (trunks >4 inches in diameter at the base):

Girdling Method – Involves a series of cuts around the entire circumference of the tree, through the bark and down to the heartwood only. In doing this you are severing the vascular bundles of the cambium tissue (located just under the bark), which would kill the top portion of the tree. You can also take an axe and make angled, spaced cuts around the trunk, again through the bark and down to the heartwood only in a method called “hack and squirt” or “tree injection method.” Quite frankly I don’t generally recommend tree injection or hack and squirt using an axe because, depending on how hard and hardy the tree structure is, the axe may glance off of the tree and increase the likelihood of injury. And suckers, fast-growing stems that grow up from the roots, are likely to sprout up.

Siberian elm seeds are tan and papery when dried. Photo credit M. Thompson

For mature trees and established saplings:

Cut and Stump Method – The premise of cut and stump is similar to that of girdling, only you cut the tree/sapling down in its entirety leaving only the stump. Just like with girdling, the herbicide can be applied directly (spray or paintbrush) over the cambium tissue just inside the bark on the outer ring of the stump. There is no point in applying the herbicide over the entire top of the stump because there are no vascular bundles in the heartwood to carry the herbicide down to the crown and root areas; applying herbicide only to the vascular tissue further minimizes herbicide inputs and localizes the application directly to the correct area.

Herbicide timing and application – One of the most important things to remember with both girdling and cut and stump methods of tree control with herbicides is that you must make the herbicide application immediately after making the cut. You literally have about a 15-minute window before the vascular bundles begin to close off and try to seal, which severely limits the amount of herbicide that makes it to the roots (where it needs to go). As a result, it is more effective to have someone making the cuts and another person following behind to immediately apply the herbicide to the cambium tissue. By making applications to only this ring, the herbicide only impacts the tree onto which it was applied. Surrounding trees with underground root systems that intertwine with those of the target tree will be safe from these applications as long as the spray, or dripping herbicide from the paintbrush, doesn’t come into direct contact with the other desirable trees.

Suckers – Wait until the sucker develops a leaf or two (but when the bark is still young and green), then apply the same herbicide(s) that you used for girdling or cut and stump directly onto the leaves and the top of the bark, down the entire length of the sucker.

Seed escapes – If you have a great deal of weedy tree seeds that have escaped onto your property, effective methods of removal can be raking them up, mowing over and bagging the mulch, or utilizing a cordless shop vac to go out and suck up the seeds on the surface before they have a chance to germinate.

The younger these plants are when you try to manage them as they invade your property, the more effective your management efforts will be and the fewer times that you will have to revisit the same sapling over and over again for control. The more mature the plant gets, the more difficult it is to control. So remember... the early bird catches the weed!

Guest author Dr. Leslie Beck (NMSU Extension Weed Specialist) is based in Las Cruces. Regular author Dr. Marisa Thompson (NMSU Extension Horticulture Specialist) is based at the NMSU Agricultural Science Center at Los Lunas.

Siberian elm leaves from a mature tree with little holes that were likely made by elm leaf beetles. Photo credit M. Thompson

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

Links:

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook page.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!