It takes a plant community

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?

             

Seed capital

One gardener’s spectabilis is another’s horribilis. When flowering plants make viable seeds that spread around the garden to germinate next year, you may think it’s a blessing or a curse.

            Remember that primary school unit about how seeds travel? I have examples of all the strategies in my garden this month, from burrs to wind-catching silky parachutes to fruits that co-opt birds to carry the seed to new locations. We plant for flowers, but the plants’ goal is to pass on their genes to another generation.

            Usually I like having the surprise of little seedlings popping up here and there in spring. This is the time of year when I have to decide who shall live and who shall die (or at least reproduce)--which seedheads to leave alone and which to clip out before they can drop their seeds.

            Some of my favorite purchases have turned out to be self-seeders, especially white bleeding heart (Dicentra spectabilis ‘Alba’). Spectabilis translates as “worth looking at,” and that perfectly describes the glowing heart-shaped flowers when they bloom in my May garden. 

White bleeding heart--worth looking at

Other enthusiastic self-seeders I enjoy include columbine (various species of Aquilegia), dwarf goatsbeard (Aruncus aethusifolius), and forget-me-not (several species of Myosotis). When these sprout in unexpected locations, I let some grow, pull some out, move others to better spots, and pot a few to give away.   

Columbines sometimes cross-breed, changing flower colors in the offspring, but this dark purple has stayed true.

             Then there’s tall verbena, Verbena bonariensis, whose seedlings are legion in my vegetable beds. This odd South American self-seeder sends up a single 3- to 4-foot stem with a small lilac purple compound flower at the top. It’s a prolific producer of seeds considered invasive in the South, but not in Massachusetts yet. I bought some from a mail order nursery ten years ago, and new volunteers sprout every year.

            I keep some of the seedlings because migrating monarch butterflies used to touch down on the flowers to refuel. I haven’t seen the monarchs for a couple of years, perhaps because their population has declined drastically due to habitat loss and the use of herbicides that eradicate the milkweed their caterpillars need for food. 

Instead of monarchs, nonnative cabbage white butterflies (Pieris rapae) are visiting the tall verbena this year.

I’m growing some milkweed now in hopes the monarchs will return.

Seeds of swamp milkweed (Asclepias incarnata) getting ready to float on silky floss

            Because I patrol the beds each growing season deciding which seedlings to coddle, which to tear out and which to relocate, I maintain something close to the balance I prefer. Like ground covers that can spread unmanageably, self-seeders can be so effective that they take over the garden. At least with the self-seeders, I can say no to future spread with diligent dead-heading in late summer and fall, removing spent flowers before they set seed.

A butterfly is born!

An eastern black swallowtail butterfly, Papilio polyxenes asterius, has emerged on my potted parsley! This summer has been very encouraging. Apparently if you plant what native insects want, they'll come.

     I planted some parsley in the vegetable garden and some in a big pot on the deck, hoping to create reproductive space for this butterfly, as well to harvest some garnish. I noticed the mature larva, a large, elegant caterpillar, two week ago.

It transformed into a pupa soon afterward.

 

When I got back from vacation, it had hatched.

I saw what could have been the hatchling flying around the garden today. It declined to stay still long enough to have its picture taken, but here's a file photo showing what it looked like:

Adult female. Photograph by Donald Hall, University of Florida.