Is it Invasive or is it just Aggressive? 

This article is NOT about invasive species, but is about the strategies I use to deal with aggressive species in the garden. But first, a note about invasive species. 

Common Reed or Phrag (Phragmites australis), Lily of the Valley (Convallaria majalis), Japanese Knotweed (Reynoutria japonica), Garlic Mustard (Alliaria petiolata), European Buckthorn (Rhamnus cathartica) – these names tend to strike fear (or at least dread) into hearts of native plant gardeners.  They are all invasive species. 

What makes them so bad? All were introduced into gardens from which they then escaped into the wild. There, their ability to spread was so powerful that they soon started to exclude the local plants (and sometime even animals).  

Invasive species may spread by seed (e.g. Garlic Mustard, Phragmites, Buckthorn) or by rhizome (Japanese Knotweed, Lily of the Valley) or both. Control of the spread is difficult, at best.  


In the US, the USDA National Invasive Species Information Center ( defines an invasive species as a species that is: 

1) non-native (or alien) to the ecosystem under consideration and, 

2) whose introduction causes or is likely to cause economic or environmental harm or harm to human health. 

This harm often arises because the plant (in this case) either spreads so prolifically as to exclude all or most other plants, or it uses chemical warfare to eliminate any competition. 

In Canada, the Invasive Species Centre ( states that invasive species “kill, crowd out, and devastate native species and their ecosystems”. It goes on to state that  

“A species in considered invasive when: 

  1. It is introduced to an ecosystem outside of its native range, and 
  1. It has potential impacts on the ecology, the economy, or society in its introduced range.” 

They also point out that in order for a species to become invasive, it must “possess the ability to outcompete and overwhelm native species in its introduced range.” 

Thus, an invasive species is one that is an aggressive spreader. And though we have many native species that are aggressive spreaders – Canada Goldenrod (Solidago canadensis) comes to mind – these plants are not considered invasive because they fail to meet the first criterion of an invasive species – they are not introduced to the ecosystem. They belong here. 

The analogy I like to use is that if a foreign military power attacked one of our cities and did a lot of damage – we would say they had invaded us. But if our own military did the same damage, we wouldn’t say they had invaded (because they belonged here) but that they were, instead, being very aggressive.   

Aggressive Native Plants in the Garden 

Seed Spreaders 

In my southwestern Ontario garden, with its soft, loamy soil, a number of plants behave rather aggressively. Some of these, such as Silphium perfoliatum (Cup Plant) and Rudbeckia laciniata (Green Headed Coneflower), self-seed prolifically. These garden bullies provide great shows in the summer, but tend to outcompete everything else. The only thing I might do to control them is cut off the seed heads before the seeds ripen. Except I don’t do this because I want the seeds to provide winter nourishment for the birds. So instead, I spend a fair bit of energy and time each spring thinning out the excess plants. (These get potted up and planted elsewhere, or are sold to others so I can buy more plants.) 

I have a number of other plants that also self seed prolifically, but they are welcome in the garden because they tend to play nice with others. These include the lovely little Pale Corydalis (Capnoides sempervires), Joe Pye Weed (Eutrochium maculatum), and a couple of milkweeds – Butterfly Milkweed (Asclepias tuberosa) and Swamp Milkweed (A. incarnata). Although they pop up all over the yard, they are not aggressive like Cup Plant so I generally welcome their spread. 

Rhizome Spreaders 

The other way plants spread aggressively in my yard is by rhizome – underground “roots” that pop up plants where you least expect them.  These tend to be the more troublesome group. Some of these plants, like Canada Goldenrod (Solidago canadensis) and Star Flowered Solomon’s Seal (Maianthemum stellatum) can produce such a dense mat of roots that they eventually begin to exclude many of the other plants. These are the real trouble makers. But I have found a way to keep these aggressive spreaders in check. 

Vines and Vine-like Plants 

Not all aggressive plants spread by seed or rhizome. A couple spread by above ground runners, too. Wild Grape (Vitis spp) and Virginia Creeper (Parthenocissus quinquefolia) are two fairly aggressive woody vines that can quickly cover fences or trees if allowed to spread. Fortunately, an quick annual pruning is all that’s needed to keep them in check. 

A perennial, vine-like plant that likes to take over garden spaces is the lovely Virgin’s Bower Clematis (Clematis virginiana). It seems that every leaf-node that touches the ground on this rapid growing and sprawling plant wants to set down roots and sprout a new plant, which then sends out its own runners (stolons) which do the same. My Clematis grows along the fence that bounds one of my flower beds. Each spring I have to follow the runners throughout the flowerbed and pull up dozens and dozens of plants. Fortunately, these are readily potted up for resale so I can buy even more plants (or to give away if you’re not as mercenary about it as I am).  

Controlling the clematis is a challenge in my garden because I like the look of it sprawling along the fence. If I wanted to keep it under better control, I would simply plant it in front of a trellis and prune it to stay there. 

Controlling the Spread of Rhizomes – Root Barriers 

My first root barrier was for Purple Flowering Raspberry (Rubus odorata). I was warned that these shallow rooted shrubs send out a lot of rhizomes, so I purchased a length of aluminum from a company that makes eavestrough. (They were installing new eavestrough on the house across the street so I just went over and asked if I could buy some.) This allowed me to “fence off” a large area.  

It took many years before the leaf litter and wood chips got deep enough to allow the roots to go over the barrier. A little maintenance once every few years would have prevented this, but where it was growing I wasn’t too worried about it. And it’s definitely an easy fix if I decide to bring it back under control. 

In order to keep plants more contained, I now grow all my aggressive spreaders in a large pot in the ground – in my case I use half of a plastic 45 gallon barrel, sunk into the garden. The barrels are cheap – you can usually pick up one for $10-20 (which gives you two pots). Sometimes you can even find them for free as I did when I volunteered to help clean up the river bank with our local conservation authority.  

After cutting the barrel in two, I then drill some 1” holes in the bottom for drainage. I dig out a hole deep enough so that only about 1” of the barrel is above the ground, set the barrel in, then put the soil back in. (This is a great opportunity to amend your garden soil if, for instance, you are putting in an acid soil loving shrub, of if your soil is heavy clay and your plant wants a sandy loam, etc.) 

I planted my Jerusalem artichokes in a half barrel 5 years ago. It has never escaped. And each fall I simply harvest as many of the tubers as I find – I always miss a few tiny ones – and the next year these missed pieces become a new crop. 

If digging a 3’ by 3’ hole in your yard is more than you can handle, I have successfully grown Common Milkweed and Grass-leaved Goldenrod in a large plastic pot – the kind small trees often come in. They’re only about 12-16” tall and a foot across. The plants grew in them for years. 

A List of Troublemakers 

The list below is by no means comprehensive, but it includes those that are the worst offenders in my garden for spreading by rhizomes. These are plants that need a lot of room to spread and may not be suitable for smaller garden spaces unless their control is kept in check with some form of root barrier.  (Listed in alphabetical order by scientific name.) 

Canada Anemone (Anemonastrum canadense

Common Milkweed (Asclepias syriaca

Virgin’s Bower Clematis (Clematis virginiana

Grass-leaved Goldenrod (Eurybia divaricata

Jerusalem Artichoke (Helianthus tuberosus

Star Flowered Solomon’s Seal (Maianthemum stellatum

Obedient Plant (Physostegia virginiana

Prickly Gooseberry (Ribes cynosbati) 

Smooth Wild Rose (Rosa alba

Purple Flowering Raspberry (Rubus odorata

Canada Goldenrod (Solidago Canadensis)  

Prickly Ash (Zanthoxylum americanum) 

You CAN grow many of these aggressive plants in a small space without them getting out of hand. I hope these suggestions help you grow some of the lovely native plants we have that you haven’t grown before because of their aggressive nature. 

Happy Native Plant Gardening.