Sometimes it feels like we’ll never get ahead of opportunistic nonnative plants. Especially with woody plants like Japanese knotweed (Fallopia japonica), it seems once they’re in your garden, you may never see the end of them.
In his book Gaia’s Garden, Toby Hemenway argues that opportunistic plants grow where we’ve given them an opening in disturbed ground and will naturally die off as stable plant communities take over (Hemenway prefers the term opportunistic to invasive, which he considers too emotionally loaded. I’m using his terminology). This is a nice idea, but considering how much Japanese knotweed I see in the course of a week’s driving, I wondered how likely Hemenway’s scenario was in my area.
|This Japanese knotweed's underground rhizomes could be as much as 60 feet long. It propagates by seeds but is also capable of regenerating from root or stem fragments.|
This week I was pruning dead branches out of a dwarf mugo pine, (Pinus mugo 'Paul's Dwarf') planted eighteen years ago that is now partially shaded by a nearby magnolia. It occurred to me that this could be an example of Hemenway’s point. The branches that were in full shade were dead, with clinging brown needles, whereas the side of the little tree that was still in sun was looking good and producing green needles. I hadn’t shaded out this pine on purpose, I’d just failed to imagine how big the two trees were going to grow and planted them too close together.
|Mugo pine reaching for sunlight outside the shade of a taller magnolia|
If the magnolia had cast its shade over an unwanted shrub like Japanese knotweed, I suppose it too would have dwindled. The problem, Hemenway points out, is that suburban landscapes, even our parks, offer so many sunny edges that favor fast-growing plants like the knotweed.
Of course, shade isn’t the only condition that discourages opportunistic plants. The goal is to replace a plant that you don’t want with one, or ideally some, that you do. If you just remove the offender, you leave open soil for opportunists to take over again. Growing together, a group of native plants that have evolved to cooperate can succeed in grabbing the resources an opportunist needs to survive, not just sunlight but also water, soil nutrients, and room to grow.
While I wait for my growing collection of native plants to knit themselves into anti-opportunist communities, I’m still cutting a couple of knotweeds at the fence line to the ground every couple of weeks, hoping they’ll eventually run out of stored energy and give up.
|I cut it down, but this deceptively frail-looking Japanese knotweed, center, keeps coming back up. Will encroaching smooth Solomon's seal help to curb its growth?|