Gardener Beware: Invasive Garden Plants, Part 1

Though I loath to admit it, the Goutweed does look rather nice in this island bed.

Not every plant that your find in a nursery or garden centre is well-behaved. Retailers often sell plants that many consider problematic or invasive. 

Why sell them then?

Not everyone would agree on what constitutes a "problem" plant. Based on my own personal struggles, I happen to think that Goutweed, Aegopodium podagraria 'variegatum' is pure evil, but I know at least one friend who thinks that it has nice variegated foliage and likes to have this plant in his garden. 

Goutweed in behind some hostas.

 Sweet Woodruff

 Sweet Woodruff taking over a part-shade garden.

On the other hand, I happen to like the little white stars of Sweet Woodruff, but I know another garden writer who felt the need to write a public service announcement warning gardeners about the this groundcover herb. Certainly, if you bring Sweet Woodruff home and have no idea how it is likely to behave, you can find yourself in a bit of a mess.

For me a problem plant is not just aggressive, it is also a plant that is hard to remove where unwanted. Vigorous perennials like Goutweed can send out roots that spread underground in many directions. Eradicating it can be very difficult. Even if you dig out the main plant, any roots segments you miss are capable of producing a new plant.

Other plants like False Lamium, Lamium galeobdolon 'Florentinum' (read more about different types of Lamium here) send out runners above the ground (similar to those of a strawberry plant) that take root and create new offspring. 

In a somewhat similar fashion some vines, as well as climbing up, will send out runners along the surface of the ground. New plants will naturally layer along the length of the stem.

I personally don't mind some Feverfew, but have to remove many unwanted seedlings.

And then there are the prolific self-seeders! Feverfew, Tanacetum parthenium seeds itself everywhere and can easily crowd out other perennials. The seedlings are easy enough to remove, but if you resent yet another chore, you may find that this plant is a real nuisance.

The best thing a gardener can do is to avoid problems with invasive plants in the first place. Here are a few suggestions to help:

• When selecting an unfamiliar plant, ask nursery or garden centre staff for references: "Is this plant aggressive or invasive in any way?" Most well-trained staff will warn you about potential issues.

Bugleweed, Ajuga can easily get out of control in moist soil

• Pay heed to descriptives. A "groundcover" will spread out more or less aggressively to cover a wide area. The Sweet Woodruff, mentioned earlier, can blanket a large shady area. If that's not what you are looking for, try to find an alternative that is "clump-forming." 

• Take note of the manner a plant spreads and how quickly it does so. "Spreads by creeping rhizomes" means the plant will travel underground. "Prolific self-seeder" may be an issue, if you dislike removing unwanted seedlings.

• Proper botanical names are an invaluable way to identify plants, but common names do have their uses. If a common name includes the word "weed" (Goutweed is a good example) someone probably gave it that name for a reason.

Creeping Jenny threatening to choke out the herb Sweet Cicely.

• The internet is an amazing resource. Before you plant something that is unfamiliar, look it up online. Type something like: "Is Creeping Jenny invasive?" into the search engine of your choice. If you get a long list of results, I'd think twice about planting Creeping Jenny.

• Make sure a particular plant doesn't have an invasive plant alert for your region. Some plants are fine in one part of the country, but can be a problem in other regions where growing conditions are very favourable. Again the internet is a great research tool.

• I am a plant collector that loves unusual things, but I have learned the hard way not to take chances. If I've looked the plant up, but still have a few lingering suspicions, I put it in a spot where I can keep an eye on it and remove it if necessary (i.e. my raised nursery bed). It often takes a year or more for a plant to establish itself. If it is going to spread wildly, you may not see it until the third year. Only after a plant passes a probationary period do I put it out in the main garden.

Popular ways to Contain an Aggressive Plant

A few words on some of the common methods for restricting the spread of a plant.

Method 1: Use an aggressive plant in a container planting.

This works, but even so, I advise you to do this with a little caution. Trailing plants like Creeping Jenny look great in a container planting, but keep an eye on it. In a shallow pot, it can cascade right to the ground and take root. 

False Lamium 'Variegatum' trailing out of an urn. Notice it has almost reached the ground. Trim it back and it would be fine for the rest of the gardening season.

I almost had this happen with False Lamium 'Variegatum'. It trails nicely, so planted it in the window box under my kitchen window. A few weeks later, I noticed its yellow flowers had begun to set seed. The little black seeds were in danger of dropping into the garden below, so I trimmed the flowers off. Then a month later, I noticed that the runners, which made such a pleasing cascade over the edges of the window box, had reached down almost 4 ft to the ground and were about to take root. I was both dismayed and impressed with the plants determination to create offspring.

Artemisia 'Silver King'

Method 2: Put an invasive plant into a plant pot and submerge the pot in the ground.

Personally, I have found this doesn't work very well. Trailing plants will skip over the rim of the buried pot and take off into the rest of the garden (I had this happen with Oregano). 

Plants with deep roots can also sneak out the drainage hole in the bottom a buried pot. 

I tried planting Artemisia 'Silver King' in a buried pot only to watch it layer itself into the surrounding garden (layering occurs when an upright stem bends down to the ground and takes root).

Pachysandra covering a large area under a tree.

Method 3: Create a deep edge or trench around an island bed that contains an aggressive plant. This works to a degree, but you would really want to make sure the edge is deep and wide, so an invader can't jump across the divide. The only other worry might the possibility of the the plant self-seeding into other areas.

Method 4: Plant a spreader into a raised bed.

Where there is a botanical will, there is a way. In the picture below, you can see that Gooseneck Loosestrife (white flowers) has spread from the raised bed to the ground below.

I've also heard horror stories where the roots of really vigorous plants like Bamboo have cracked through concrete and escaped into the surrounding landscape.

Bottom line: know what you're planting and how it is likely to behave. 

Too often gardeners are impatient to fill up their flowerbeds and choose a plant that will spread quickly. Patience is a virtue, especially when it comes to gardening.

Read Invasive Plants Part 2 and Part 3.