Wednesday, March 15, 2017

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.

More on the subject of invasive plants, in part two.

15 comments:

  1. I love ajuga and sweet woodruff and they are crazy rampant!! I made the mistake of using creeping jenny (it was in a planter and I put it into the ground in the fall a few years ago - GAH!!!!!)

    ReplyDelete
    Replies
    1. We all make mistakes. When we moved in I had no idea how aggressive Lily-of-the-Valley was. Now I know and how!

      Delete
  2. I didn't know feverfew fell into this category. I read that it self seeds, and felt that was a bonus - as in the way my nigella and poppies self seed. After seeing a photo of roses surrounded by feverfew, I had planned to do just that this year. Back to the drawing board. Many thanks for the heads-up!

    ReplyDelete
    Replies
    1. I think I gave you the wrong impression Annie. It does self-seed very prolifically. Some gardeners would find that annoying. I like Feverfew. It is a chore to remove seedlings, but it looks so pretty I personally don't mind the extra work. If you like self-seeders like poppies and nigella you will probably like Feverfew. One caution. It can as I wrote crowd other plants. If you plan to grow it with roses you may have to thin it to give the roses room to breath. One last thing. Feverfew hates being moved. It does best wherever the seed happens to land.

      Delete
  3. The previous owners of the house we bought three years ago loved so many of the aggressive plants. I've got ajuga, goutweed, privet, periwinkle, honeysuckle, creeping jenny, rose of sharon, and english ivy. I'm slowly conquering the goutweed and the english ivy. The creeping jenny is planted in an inhospitable area (thank goodness), so it's self controlled. The honeysuckle I pull, and privet and rose of sharon... well we cut down it wherever it springs up unwanted as there's too much to take it all out (an entire 50 foot hedge of privet). Periwinkle and ajuga I live with. There are only so many battle fronts I can fight! (I haven't even mentioned the creeping charlie...)

    ReplyDelete
    Replies
    1. Gosh you have your work cut out for you Kathleen. I've tried to eliminate the Goutweed I inherited, but am still struggling with it in a couple of spots. Like you, l live with a few other invasives that I inherited from the previous owner. The worst thing I found was Knotweed. Almost 15 years have passed and a little bit still tries to come back ever year.

      Delete
  4. I have a problem with feverfew, also sage and mint, herbs can be very invasive....crocosmia is also the bane of my life along with ivy! An interesting post.xxx

    ReplyDelete
  5. I guess there are some advantages to gardening in poor, sandy soil in Zone 4! Feverfew is a treasured plant for me and I would not call it invasive. It does reseed, but not aggressively. It is one of my favorite plants for bouquets. Ajuga spreads very slowly where I have it under some shrubs. I planted sweet woodruff under shrubs last year and am hoping it will spread. I had it once, but it all died out. Chinese lanterns, gooseneck loosestrife, Silver King artemesia, and yarrow millefolium are all banished from my gardens however. Everyone just needs to figure out the thugs for their region. I like your idea about having a "probationary" bed to try new things out in before they enter your flowerbeds. Sometimes I'm just not patient enough or realistic enough!

    ReplyDelete
    Replies
    1. I like Feverfew too Karen! Perhaps I should edit the post. What I meant to say was that, while I like Feverfew, some gardeners would find its tendency to self-seed prolifically annoying. I believe Ajuga needs somewhat moist conditions to really become a bother and there are some varieties that are less aggressive.Using Sweet Woodruff under a shrub would be nice (in my opinion). In other settings, it may be too aggressive. I agree with your list of banished plants too!

      Delete
  6. Excellent post Jennifer! Ajuga Burgundy Glow makes a wonderful groundcover and doesn't get too crazy here in my zone 7 garden, but Creeping Jenny is another story! Creeping Jenny is a nice looking plant, but I would never put it into the ground here. It is great in planters, where it can be kept under control, and cascades nicely. Another plant to mention is Houttuynia cordata (Chameleon Plant). I had an encounter with Chameleon Plant some years back. I had purchased some of the lovely variegated plant with shades of gold and pink its its foliage, only to quickly realize that it multiplies vigorously! It took several years of pulling it out by the rhizomes to completely eradicate it, and now whenever anyone mentions how interesting Chameleon Plant is, I simply reply...run!

    ReplyDelete
  7. Oh THE LAMIUM....did you have to remind me. Here I was, in my pretty little cloud of spring longing, looking forward to the dear snowdrops and tiny Cornus mas blooms, and there was lamium, right there in your wonderful post. Sigh... I must really do something about it this year... yes I will, although it is pretty, I will give it that. Happy Spring Jennifer. xo

    ReplyDelete
    Replies
    1. Sorry for bringing up a sour spot so early in the season Brenda. Tackling an invasive weed is a huge task, isn't it. I still have some goutweed and lily-of-the-valley to tackle this spring. Happy spring to you too!

      Delete
  8. "Spreads quickly by rhizomes" is always a clue to me--I don't want that plant!! I think we've all made this mistake. I remember using the trailing vinca (forgotten the botanical name) quite often in containers. My friend also used it, and it spread to the ground where she had an awful time eradicating it. Then again, as you say, there are some invasive plants you don't mind. I like Sweet Woodruff and find it easy to pull out clumps spreading where I don't want. I wish someone had told me, though, about Rudbeckia triloba, before I let one volunteer plant go to seed--I have dug out more seedlings than I care to think about!

    ReplyDelete
    Replies
    1. Oh, oh I recently introduced some Rudbeckia triloba into my nursery bed. I didn't know it was a prolific self-seeder either Rose.

      Delete
  9. Sweet Woodruff can be aggressive but I find very easy to control...since its easy to pull out. The one vine I wish I never planted was Virginia creeper I've tried to eradicate it for years with no success...Creeping jenny I I transplanted to containers and makes it through our winters here. Two shade lovers that can be invasive in my yard are Gooseneck Loosestrife and Chelome but also easy to pull out... can't wait to get out in my gardens....

    ReplyDelete

I love to hear from you. Thanks for leaving a comment.