If you’re a gardener, there is an excellent chance that you’ve planted, at least once in your life, an ornamental perennial that turned out to be a thug, leaving its allotted place in the garden to take up a larger and larger swath of territory year after year. Over time, some of them become so invasive they smother all neighboring plants to form a monoculture. And controlling them can be a pain.
I find it unfortunate that so few plant merchants point out the weedy nature of these plants. Because if you know about the problem, there’s a lot you can do to prevent it. Such as not buying the plant to start with… or planting the thing in a spot when its invasiveness doesn’t really matter, perhaps in the far corner of your lot when you never intend to garden seriously. And you can also plant wandering perennials inside a barrier… at least those with rhizomes.
Rhizomes are underground stems that grow sidewise and produce offsets here and there along their length. The longer they are, the farther the plant will wander. The neat thing about rhizomes is that you can control them quite readily. Just insert a rhizome barrier into the ground and plant the invader inside it. Of course, this has to be done before the horse is out of the barn, that is, before it has taken over your garden!
There actually are commercial rhizome barriers made of thick black plastic that you can purchase (suppliers of bamboo plants usually offer them). For most perennials, though, you don’t have to look that far. You’ll have just as much success using a plastic bucket or large plastic pot as a barrier. Simply remove the bottom (for drainage purposes) and insert the barrier into the ground. I like to leave about 2 inches (5 cm) of the barrier exposed to stop any rhizomes that might try to wander out and over the barrier, especially over time as the soil builds up.
Deep barriers for Deep Rhizomes
The barrier has to be deep enough to stop the rhizomes from spreading. That’s easy with smaller perennials, as in general the depth at which the rhizomes grow is roughly proportionate to the height of the plant. An 8-inch (20 cm) barrier will readily stop most low-growing plants grown as groundcovers, like sweet woodruff (Galium odoratum) or winterberry (Gaultheria procumbens). You’ll need a deeper barrier, reaching down about 12 inches (30 cm), for medium-height perennials like beebalm (Monarda spp.) and Chinese lantern (Physalis alkekengi franchetii). To stop bamboos, traditionally a 24- to 30-inch (60-75 cm) barrier is used. You want to stop the worst thug of all, Japanese knotweed (Fallopia japonica)? You’ll need a barrier 5 feet (1,5 m) deep (you might want to use a 5-foot length of culvert pipe standing upwards). Good luck digging a hole for that!
Higher Barriers to Stop Runners
Certain weedy ornamentals produce runners as well as or instead of rhizomes. Runners, also called stolons, are horizontal stems that creep above the ground rather than below. They are not as easy to stop as rhizomes, as they tend to wander right over barriers, at least occasionally. Among plants with wandering runners are mints (Mentha spp.), periwinkle (Vinca minor), strawberries (Fragaria spp.), and bugleweed (Ajuga reptans). For these plants, not only is it very important to leave 2 inches (5 cm) of barrier above ground (see above), but also to check occasionally, pruning off wandering stems if needed.
Of course, rhizome barriers will only work on plants that wander sideways. They will be of no help whatsoever for perennials who’s thuggishness is due to aggressive self-sowing. This group includes many of the ornamental thistles (Echinops, Onopordum, Cirsium, etc.), biennial evening primrose (Oeonothera biennis), most mallows (Malva spp.) and anise hyssop (Agastache foeniculum). For these wanderers, keeping the ground covered with mulch will keep them from getting out of bounds, as seeds have a hard time sprouting through mulch.
The following perennials all have wandering rhizomes and/or stolons and can become thugs, at least under some circumstances. Be careful where – and how! – you plant them!
- Achillea millefolium (common yarrow) zone 3
- Achillea ptarmica (sneezewort) zone 3
- Aegopodium podagraria (bishop’s goutweed) zone 3
- Ajuga reptans (bugleweed) zone 4
- Anemone canadensis (Canada anemone) zone 2
- Anemone hupehensis, A. x hybrida (Japanese anemone) zone 4
- Arrhenatherum elatius bulbosum ‘Variegatus’ (bulbous oatgrass) zone 4
- Artemisia ludoviciana (silver wormwood) zone 2
- Artemisia stelleriana (hoary mugwort) zone 2
- Asarum spp. (wild ginger) zones 3-6
- Bromus inermus ‘Skinner’s Gold’ (golden brome grass) zone 3
- Campanula glomerata (clustered bellflower) zone 2
- Campanula punctata (spotted bellflower) zone 3
- Campanula rapunculoides (creeping bellflower) zone 3
- Chrysanthemum rubellum ‘Clara Curtis’ (Clara Curtis chrysanthemum)
- Convallaria majalis (lily of the valley) zone 1
- Dactylis glomerata ‘Variegata’ (striped orchardgrass) zone 5
- Dicentra formosa (Pacific bleeding heart) zone 3
Duchesnea indica (Indian strawberry) zone 5
- Euphorbia cyparissias (cypress spurge) zone 4
- Euphorbia amygdaloides (wood spurge) zone 6
- Euphorbia griffithii (Griffith’s spurge) zone 5
- Fallopia japonica (Japanese knotweed) zone 3
- Filipendula spp. (queen of the prairies) zone 3
- Fragaria spp. (strawberry) zone 3
- Galium odoratum (sweet woodruff) zone 3
- Gaultheria procumbens (wintergreen) zone 2
- Glyceria maxima ‘Variegata’ (variegated reed sweet-grass) zone 5
- Helianthus spp. (perennial sunflower) zone 3
- Helianthus tuberosus (Jerusalem artichoke) zone 4
- Hemerocallis fulva (tawny daylily) zone 3
- Houttuynia cordata ‘Chameleon’ (chameleon plant) zone 4
- Kalimeris spp. (kalimeris) zone 3
- Lamium galeobdolon (yellow archangel) zone 3
- Lamium maculatum (lamium, spotted deadnettle)
- Leymus arenaria ‘Glauca’ (Lyme grass) zone 4
- Lysimachia clethroides (gooseneck loosestrife) zone 3
- Lysimachia nummularia (moneywort) zone 3
- Lysimachia punctata (yellow loosestrife) zone 4
- Macleaya spp. (plume poppy) zone 3
- Matteuccia struthiopteris (ostrich fern) zone 3
- Mentha spp. (mint) zones 2 à 5
- Miscanthus sacchariflorus (Amur silver-grass) zone 2
- Monarda spp. (beebalm) zone 3
- Onoclea sensibilis (sensitive fern) zone 3
- Pachysandra terminalis (pachysandra, Japanese spurge) zone 4
- Persicaria affinis, formerly Polygonum affine (knotweed) zone 3
- Persicaria amplexicaulis, formerly Polygonum amplexicaule (fleeceflower) zone 4
- Persicaria bistorta, formerly Polygonum bistorta (bistort) zone 3
- Petasites japonicus (butterbur) zone 3
- Phalaris arundinacea ‘Picta’ (gardener’s garters) zone 4
- Phragmites australis (common reed) zone 3
- Phyllostachys, Pleoblastus, Sasa, etc. (bamboo) zones 5-10
- Physalis alkekengi franchetii (Chinese lantern) zone 3
- Physostegia virginiana (obedience plant) zone 3
- Polygonatum spp. (Solomon’s seal) zone 3
- Ranunculus repens (buttercup) zone 4
- Rudbeckia laciniata (cutleaf coneflower) zone 3
- Solidago canadensis (Canada goldenrod) zone 2
- Spartina pectinata ‘Aureomarginata’ (gold-edged prairie cord grass) zone 4
- Stachys byzantina (lamb’s ears) zone 3
- Symphtyum spp. (comfrey) zone 2 or 3
- Tanacetum vulgare (tansy) zone 3
- Tiarella spp. (foamflower) zone 3
- Trifolium repens (white clover) zone 3
- Vinca minor (common periwinkle) zone 4
- Viola rivinia ‘Purpurea’, syn. V. labradorica (Labrador violet) zone 4
- Waldsteinia spp. (barren strawberry) zone 3
Invasive Shrubs and Trees
Some shrubs and trees can also be invasive. Here is an article about them: Trees and Shrubs that Get Around.
Great blog article! Can you recommend any aggressive growers for an eroding creek bank that is almost vertical in areas? The soil is a bit sandy by the creek and this is a woodland area with morning sun (zone 7), far from our gardens and between has a meadow-y lawn that I mow regularly where any spreaders will get mowed down if they make it say more than 10 feet from the creek bank. I planted a weeping willow and 2 different pussy willows but want aggressive, stabilizing groundcover or taller helpers for the surface as well as that might be able to climb or run down the cut bank to help hold.
Pretty much any of these plants will help. Most already are used for that purpose in areas where their invasive nature is not an environmental threat.
We are considering installing a 24-inch deep rhizome barrier for our pachysandra areas. I didn’t know that bee balm was invasive, too – good to know!
Well, at least bee balm can be pulled out fairly successfully.