Purple Loosestrife

Purple Loosestrife

Part of Ep. 703 Invaders

Learn more about the invasive plant, purple loosestrife (Lythrum salicaria).  Botanist David Kopitzke explains why this perennial is such a menace--and illegal in Wisconsin.

Premiere date: Jul 10, 1999

TRANSCRIPT+
Wisc Gardener Transcript: 

Shelley:
Up to now, we've been talking about some wonderful but aggressive plants that really can serve a purpose in your back yard. Now we're going to look at one that you should never grow. We're talking about purple loosestrife, this vast sea of brown behind me. This plant is illegal in Wisconsin. I'm with botanist David Kopitzke, of the endangered resources bureau. David, let's start out with a really close look at this, so we know if we see it.

David:
Okay, this is what the plant looks like in June. It's called Lythrum salicaria. Purple Loosestrife is its common name. And you can see that it has leaves without teeth. The leaves come three or two from a given point. It has ribbed stems. So, you can see six sides on this stem.

Shelley:
It's very distinctive.

David:
Right. Coming from this stem in midsummer is a flower stalk. In the garden, it's only something like 18 or 24 inches tall. But here in the wetland, it gets to be eight feet or so tall. A spike of really beautiful purple flowers is then followed in late-summer by seed heads producing, literally, thousands and thousands of seeds.

Shelley:
Now, I've seen a lot of tall, purple blooming plants earlier in June or May, I think. Would that also be purple loosestrife?

David:
That would be something different. There are other wildflowers, other plants, that grow and blossom in June. And there are still others that are purple and blossom along roadsides in September. But those would be different plants. Those wouldn't be purple loosestrife.

Shelley:
Did this purple loosestrife get planted here on purpose?

David:
No, nobody planted this particular plant on purpose. What they did do across the road from where we're standing is plant a few plants of purple loosestrife, this would've been about 30 years ago. And at that time, this wetland behind us was a wonderfully diverse area of grasses and sedges and wildflowers and shrubs, a wonderful place for a variety of wildlife.

Shelley:
Now, as far as the eye can see, I see just these brown seed heads. This is all purple loosestrife?

David:
That's right, all the way to the base of the hill in the distance. Dozens of acres of purple loosestrife have come from just a few plants that were planted by a perfectly well- intentioned gardener.

Shelley:
How does this spread so fast and so wide?

David:
Let me show you. I'll cut a seed head over here and I'll show you how tiny the seeds are. This will surprise gardeners, to see this plant eight feet tall, but that's what happens to purple loosestrife when it's in a wetland. I'll break apart some of the seed heads, here, and you can see these tiny seeds blowing about in the wind.

Shelley:
They look like dust.

David:
Exactly. And they spread so easily because they're so tiny. They spread by wind. They spread by rain. They spread by flowing water. They spread by feet, too. So, we'll have to wipe off our boots when we leave here. It's even spread by deer. You can walk into the wetland right here along the deer paths and you can see them flanked on either side by purple loosestrife plants planted inadvertently by those deer.

Shelley:
Just from the seeds falling off of them and rooting wherever they are.

David:
That's right, in the mud on their hooves.

Shelley:
That explains why it might be illegal in Wisconsin. Is it a statewide problem?

David:
It is. You'll find purple loosestrife in virtually every county, many counties throughout the state. So, it isn't right here at this one spot, but throughout Wisconsin.

Shelley:
With it being illegal, I assume we don't have to worry about mistakenly purchasing it.

David:
Well, I wish it were that way. But you can still find, in seed catalogs, purple loosestrife for sale, but sometimes under other names. You'll see it under morden red or morden pink or wild rocket. And that's the reason you should be careful to look for the latin name, Lythrum salicaria-- purple loosestrife. That's the one you should not grow.

Shelley:
I've also heard that some of these varieties in catalogs are sterile. Then they're safe, right?

David:
That was first thought to be the case. But we've since discovered that those so-called sterile varieties will cross with other varieties and produce perfectly viable seed that then can grow and invade wetlands. So, even the sterile variety should be avoided.

Shelley:
So, if I really want to help the wetlands and the wildlife in them, if I've got this in my back yard, I should dig it up and destroy the plant.

David:
Right. That's the simplest way if you've just got a few plants in your garden, to dig it out and put it in the compost pile. You can also cut off the flower heads if you've got more. That will prevent it from going to seed. You can spot treat stems. You don't want to spread herbicide over a large area. Just individually, stem-by-stem, cut the stem and put the herbicide on it. And the Department of Natural Resources is experimenting with control by insects, bio-control. Four different insects are being introduced into Wisconsin wetlands to control this plant.

Shelley:
That's great. So there is hope for us.

David:
That's right.

Shelley:
Thanks, David. Here's how you can get more information on purple loosestrife, controlling it and other problem plants in Wisconsin.

Well, as we've shown you, many invasive, aggressive plants can serve a great purpose in your garden. There are a few to avoid entirely, however. The key is to know your plant, know its characteristics and know how it's going to behave-- BEFORE you put it in the ground. And think twice before accepting strange plants from well-meaning gardeners, especially if they say something like, "Oh, this? It'll grow anywhere." I'm Shelley Ryan. Thanks for joining me.

Did this move? I could've sworn it was-- nah. Thanks for watching the Wisconsin Gardener.

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