Invasive Plants

It’s been a fabulous spring of relentless shopping, which has left me broke and facing a lot of empty containers in the garden. So I’ve decided to save a little money by pulling out the pugnacious jokers (a.k.a. wildly invasive plants) and popping them into pots.

Invasive plants, unless they are native, should never be planted on country properties. They can knock out local populations of plants without a thought. Look at what purple loosestrife has done in wetlands. You should never let anything but native plants go ramping about the countryside.

Though aggressive by nature, most invasive plants are good in their way. It’s possible to keep an eye on them in containers and apply that heavy controlling hand when needed. The following travel by stolons or rhizomes:

  • Ageopodum variagatum (goutweed). I would never grow this, and those who do are now bemoaning the fact that they can’t get rid of it. It will invade every nook and cranny and pop up huge distances away from the mother plant. Every time you pull on this plant, its root system will break up and make new plants. But in a container, this is a beauty. It will survive very nicely in a shady spot, but, please, please, don’t let it go to seed. It will ravage a neighbourhood.
  • Macleaya cordata (plume poppy). This is a gorgeous plant that not only threatened to take over my garden but my neighbour’s as well. It probably will behave properly if it’s hemmed in, in shade and in terrible soil. Right now, I have it as a centrepiece in a huge pot, and it’s stunning, with large dove grey felted leaves. It grows to about two metres.
  • Petasites japonica. Another giant that looks great in a container. It has huge (60-centimetre) leaves with serrated edges. It is the best indicator plant I know — that is, it wilts before anything else and will tell you when to water. In a container, it looks fabulous, especially if you want to get a little tropicalismo going in the garden.
  • Geranium macrorrhizum. I feel so ambivalent about this plant. I want to yank it out by the armloads. It has the most awful magenta blooms, but the foliage has a strange and wonderful scent with great autumn colour. A good erosion plant and even better in a big old tub.
  • Cerastium tomentosum (snow-in-summer). Its glorious silver foliage with starry white blooms will take over a hillside if you let it. Stick it in window boxes and let it drip over the sides.
  • Lysimachias (loosestrifes). This is not to be confused with purple loosestrife, which is a whole other family. Lysimachia ciliata (bronzy foliage, yellow flowers) and L. cletheroides (fabulous white blooms with a whimsical way of nodding to one side) are both terrific when you first acquire them. What a pain when they decide to stay. They will crawl everywhere but look sensational in a big container together or alone.
  • A. ludoviciana ‘Silver Queen’ and ‘Valerie Finnes’ (Artemisias). I am mad for all artemisias but must admit that these two go bananas, and it can be boring to root them out. They serve as good background plants for other pot plants and will actually survive winter in a large pot or window box.

The following are the rampant seeders. Having them in a container means keeping an eye on them and never letting them go to seed. I was given a hawkweed or cow parsley, Heracleum, a few years ago. Last year, it reached its full size: about three metres high and more than two wide. It was hysterically funny, strangely exotic but did fill an impossible spot with morning sun. I kind of forgot about it, to my regret. It has white umbels 60 cm across, and each one has a thousand seeds guaranteed 99 per cent germination. I have millions of seedlings scattered all over my garden. It’s my summer job to rogue them out.

  • Cryptotania japonica makes a lovely purple shadow under a shrub. But there’s no doubt that it is a terrible seeder. Someone called it the goutweed of its species. I’m still pulling out seedlings from a plant gone to seed years ago. But in a pot, it is an attractive addition to deck or balcony.
  • Geranium pratense has a lovely pale blue flower and is one of the parents of a great cultivar, ‘Johnson’s Blue.’ You’ll know when you have this one?it will end up making a geranium plantation if you let even one plant go to seed. You can combine these plants with stunning annuals such as coleus and plectranthus and, if the container is large enough, just leave them alone for the winter. They’ll survive?which is the name of their natural game.