The Holly and the Ivy… Festive, but Not in Your Forest!

English holly plant
Two-thirds of this English holly plant is below-ground root, illustrating how hard it is to eradicate this invasive plant from Washington forests. Photo Kevin Zobrist/WSU Extension

This is an English holly (see photo) that I pulled out of the ground. What I want to impress upon you are the measurements. From the top to the root collar is about 12 inches. The root then extends another 24 inches. Folks, that’s a 2-to-1 root-to-shoot ratio. Maybe they should change the expression “tip of the iceberg” to “tip of the holly.” Can you imagine what the root system is like on larger hollies? This is one of the big problems with this invasive species—it’s so firmly rooted that anything larger than what I pulled up is very difficult to dig out and causes a lot of soil disturbance.

Most of our invasive plants can ultimately be killed by shade, so establishing a healthy forest overstory is a long-term solution in most cases. Not so with holly, as it is extremely shade tolerant. Other understory vegetation will be long gone from heavy shade before the holly gives up. It spreads not just by birds eating the red berries, but also by suckering and layering.

“But it’s so pretty!” Yes, and that’s why most of our invasives ended up here—because of their beauty. Think Scotch broom, butterfly bush, etc.

“But the birds enjoy the berries.” Yes, but the birds also enjoy a myriad of native food sources. They did just fine for thousands of years before this English import, and they will continue to do fine without holly trees. Each of those red berries represents a new infestation that will crowd out the diversity of native food sources that a whole host of different wildlife species depend on, and each of those berries represents a lot of difficult restoration work on other properties, parks, and natural areas.

Holly is dioecious, meaning male and female on separate plants. The females are particularly bad because of all the berries that feed infestations. The males are hardly innocuous, though. They fertilize the females, and they spread by suckering and layering to form large thickets.

“But I only have a few holly plants here and there and they don’t seem to be spreading.” Perhaps not above ground… yet. It may be getting well-established below ground to prepare for future expansion. Recent research (Stokes et al. 2014) suggests that a holly plant can stay relatively contained for about 14 years, after which it begins growing exponentially creating large colonies that overtake the rest of the understory.

So what to do? If you see holly sprouts that are one inch or less high, pull them out. Use gloves and pull slowly (don’t yank) with firm pressure to ease the whole root out. For bigger specimens, you may need to do some serious digging. Or if it’s not practical to dig, you may need to use an herbicide. Foliar sprays aren’t practical because the holly resists them with their thick, waxy leaves. Cut stump treatment is a common approach in which herbicide is applied to the stump immediately following cutting. A local study by EarthCorps in 2013 found that triclopyr was more effective than glyphosate for cut stump treatments.

The study also found that frilling (a.k.a. hack and squirt) with triclopyr or stem injection with imazapyr were even more effective. Stem injection is nice because the applicant and surrounding environment have minimal exposure to the herbicide. Cut stump and frilling also have pretty low impact as there is no overspray on other vegetation and a relatively small amount of herbicide is used, compared to foliar sprays. Check with your county’s noxious weed program for more information and recommendations. Make sure that any herbicide you use is registered for your type of site (forestry, for example) and always follow all label instructions.

Holly is not currently listed as a noxious weed by the Washington State Noxious Weed Control Board, meaning that control is not state-mandated and the plant can be cultivated, bought, and sold (it is on the monitor list, however). Adding a species to the noxious weed list is subject to a decision by the Board and, in some cases, the Legislature. Holly was considered for listing in 2010 and 2011, but this move was opposed by holly growers who were concerned about the resulting economic losses. Some holly growers argued that holly is not invasive, does not readily spread by seed, and is primarily growing on locations where there had previously been holly orchards. The issue also was covered by the Pacific Horticulture Society.

Here are two fact sheets on controlling English holly:

English Ivy Invades Forests

What’s wrong with a little ivy? Plenty. The photo below was taken in Olympic National Park, demonstrating that invasive species know no boundaries. This was only a small piece of the infestation, which was killing and pulling over trees and completely destroying the native understory. This has very little wildlife value, unless you’re a rat.

English holly infestation in Olympic National Park
English ivy, like this infestation in Olympic National Park, can kill and pull down trees. Photo: Kevin Zobrist/WSU Extension.

“But I only have a little!” That’s how this started out, too. It spreads aggressively both vegetatively and by bird-disbursed seeds. There are a number of different varieties, all invasive and all brought in as ornamentals.

Spraying is challenging because ivy has a thick, waxy leaf, but there are some products listed for use in this way (always follow label instructions). But it pulls up (roots and all) pretty easily, so with a little elbow grease you can make good strides. You should wear gloves and a dust mask for this, for the sake of comfort. For ivy growing up and strangling trees, you don’t have to pull it all off the tree (which could damage the bark, not to mention needless effort. Pull it off up to shoulder height, making sure to sever all stems growing up the tree. Without ground contact, the remainder up the tree will die. Then work on pulling the stuff on the ground back from the tree, pulling it up by the roots. Here are three fact sheets on controlling English ivy:

By Kevin Zobrist, Regional Extension Forestry Specialist, WSU Extension, kevin.zobrist@wsu.edu

[This article is an edited version of the article that Mr. Zobrist first published in the May/June 2015 WSU North Puget Sound Extension Forestry E-Newsletter]

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