It’s one of the most common questions we hear, voiced with level ten frustration by gardeners faced with a colony of a running-root aggressive perennial. Whether fill-in-the-blank is a weed (first group, above) or a good groundcover mistakenly placed as if it could live peaceably in a perennial bed (second group), the effect is the same. The runner spreads into every available space.
At first the invasive plant simply causes a visual blur by its omnipresence. Then it infiltrates the crowns of the rightful residents. (Preceding photos.) Desirable plants that are shorter in stature than the invader become shaded and weak. The gardener tries to intercede, digging out the unwanted plants or coating them with systemic herbicide. It’s to no avail. The invasion begins anew from bits of root that remain within the desirable plants’ root zones.
Here is what we do to get rid of such bad actors.
As our example, we chose snow on the mountain (the variegated form of all-green bishop’s weed or gout weed, Aegopodium podagraria) simply because we had just counseled a gardener on an infestation of this weed. It could as easily have been another plant on our list. The solution works. We have beaten all of the perennials in both lists using this process. However, it takes time and requires dogged determination. If you are ready to be serious about weed eradication, read on!
If the weed-infested area has desirable plants in it, we identify those we will keep. We limit the list of keepers to really special plants.
We dig out each perennial or shrub we will keep and wash it down to bare root. We divide it into pieces small enough to be absolutely sure that no weed roots remain. We do not exempt any plant from this treatment, even perennials with “do not disturb” reputations such as peony, baptisia, hellebore and goatsbeard. They can and must be divided until clean. If we skip this step the eradication will fail and the infestation will move to more beds.
We plant the clean plants into a holding bed where they can grow for at least one growing season. During that time, we keep an eye on them for any sign of the target weed within their crowns.
Sometimes there are keeper plants that cannot be moved, such as very large shrubs or well established trees. We will be digging as close to them as we can in Step Two and establishing a no-plant perimeter around them. At the end of this eradication process we will periodically inspect the saved plant’s crown and this perimeter to pluck any resurging shoots.
It’s best to be very selective of keepers from the beginning. If a shrub or perennial within an infested bed is relatively quick to grow and readily available at garden centers we almost always dig, discard, and eventually replace that plant.
Apply that selectivity to this site. It is the garden we were visiting that prompted us to compile this article using bishop's weed as illustration. All-green Aegopodium, bishop's weed, has infiltrated this bed and is expanding further left all the time. How to choose keepers?
This step can be worthwhile by saving on labor hours. However, it requires the passage of time. So skip to Step Three if you aim for the earliest possible replanting date.
In this step we smother the infested area for a whole growing season. The point of this step is to weaken the colony or keep it from spreading further until we have time to undertake a serious campaign.
We cover the entire area with a material that will block all light from reaching the weeds and last a growing season. We use newspaper in sections 4- to 5 pages thick, or yard waste bags cut open and laid flat, or an opaque tarp. Overlap paper by 6” or more. Add several inches of mulch over the top to hold paper down. If you choose a tarp, be sure it is opaque. The popular hyperlink-blue poly tarps admit light. Keep a tarp in place with bricks or other weights.
Some people smother with cardboard but we don't do this when we know we must dig the area the next year. Cardboard does not break down quickly so we would have to handle deteriorating cardboard in preparation to dig. Ugh.
Leave the smothering layer in place for an entire growing season. There's more about bed prep by smothering in What's Coming Up 95 (follow the link, download the pdf magazine and read pages 6-8) and in Smothering Reprise.
Perhaps you will choose to use herbicide - weed killer - to weaken the colony. Your choice, but we do not recommend you rely on herbicide for other steps.
Now we peel back the tarp, if we used one as our smothering layer. We are now going to slice the entire area into small rectangles and remove roots.
In this case, under the fir tree, we did smother for a year to weaken the colony but we smothered with newspaper and mulch, now degraded and we can dig right through it.
To do this we work methodically along lines that effectively stripe the entire area into six-inch wide rows. We dig with a spade – a rectangular blade, not the curved blade of a shovel - or a garden fork.
We slice down repeatedly along our first stripe, bending down at each slice to lift and sift that soil, removing weed roots from one small, deep rectangle of soil at a time. We stand on still-infested ground, insert the spade or fork, then push down and back on that tool’s handle to loosen the bite of soil forward of our cut.
We push our digging tool deep enough that any visible tips or ground-level remnants of the weeds rise up when we lean back on the tool handle. If the soil rises but the weed tops do not rise, we know to re-insert the blade deeper.
Dig as deep as the roots run. Hop over to our Youtube channel for a video demo.
When we have crossed the entire infested area we step back six inches and begin slicing stripe number two parallel to stripe one, working back toward “Go.”
This is a good time to demonstrate our determination as well as our respect for the weed’s tenacity. If our sifting fingers feel a runner snap, we chase it! We also keep track of all roots and infested plant crowns. We do not let them drop on the soil where foot traffic may press them back into the soil and into growth. We throw them on a tarp or in a bucket.
When our stripe will cross an unmovable plant's root zone, we dig as far into that plant's roots as we can, chasing weed roots into the crown. We find that it is most effective to do this with a fork, working radially in toward the trunk. Some running roots will snap. We note where this happens because in the next step we will come back looking for weed shoots emerging at those points.
If you undertake this process, study your weed's roots so you know what you are looking for and, more importantly, how to know when you are missing something. For instance, bishop's weed roots are thick and light colored in fall and early spring. So we prefer to dig it then when it is a stand-out. We can easily follow each runner to its end, which otherwise would give rise to a new shoot.
No matter how thorough we are in digging, we know some portion of the weed will remain. Bits of root were missed in digging, were running deeper than we could dig or couldn't be extricated from immovable, desirable plants. Often, runners extend out of the bed under adjacent lawn. There, mowing keeps them in check, cutting the top growth so small that it is unable to create a lot of starch to produce much new runner below ground. But if that weed within the sod can sprout a leafy shoot in the bed we just cleared, it will quickly recolonize the area.
So we return weekly to patrol for resurgent growth, especially along lawn edges and in the crowns of shrubs or trees we left in place.
If we pluck out new greenery as it forms and before it spends any time in the sun, we can starve the remnant roots. If a shoot is plucked as it breaks the surface, before it can photosynthesize and send starch to its nether parts, the root will eventually run out of starch it needs to make new shoots.
After a time, we find nothing on a weekly basis. Then we extend our follow-up interval to two weeks and then a month. One day we can declare victory, and replant with more genteel species.
Sometimes we can declare an area clear except for one or two hot spots, such as a section of lawn edge, along a fence line or within a shrub’s crown. Then we flag and revisit just those spots. This is where we stand with some of the worst weeds: at long-term truce. We gain a clean bed for the price of continual, limited patrol.
On our Youtube channel we posted a short video of follow-up on a bishop's weed project. After a year of weakening the colony, followed by one fall and one spring of digging section by section as we found the time, and one summer of following up, we can do just one patrol per year.
We have numerous arguments against herbicide use. Those that apply here are impracticality and self-delusion. Herbicides only work to the extent the leaf surface can absorb toxin. Lots of root and not so much leaf means the plant won't be killed by one application. The gardener must wait for surviving root to produce new leaf, looking all the while at plants dying back - including all the hostages - then apply weed killer again. Sometimes three or more applications are necessary, each increasingly tedious as the gardener must hunt down each remnant.
People try this and after application number two they tend to say, "That's that, then." They stop patrolling for survivors, especially if it is by then the end of the gardener's growing season... which is not the end of the plant's growing season. The weed regroups and is back in force the next spring.
In the time it takes to complete this, we can have a clean bed through our four steps.
So you noticed that the queen of running-root aggressors, bindweed (Convolvulus species), is not on our list. We use this same approach on those terrorists but we have never had a complete victory over bindweed, except when it is a new, small colony. The best we have done against a serious infestation by this native, extremely deep rooted species is to beat it back to a single spot in a garden. Then we mark the spot and visit it every month to find and remove all its new shoots, continually containing the spread.