Spare those weeds

Pulling out weeds always seems like a virtue, and letting them grow a sign of slovenliness. But it turns out that a few weedy patches promote biodiversity in the yard. 

A neighbor has let these weeds take over a stretch of curb strip

When we removed large hemlocks last year to avoid spraying them for hemlock woolly adelgid, we were left with two sunny clearings. I planted native shrubs and small trees, but it’ll be a few years before they grow big enough to fill the space. Meanwhile, lots of weeds have stepped forward to fill the open space.

A stand of what I think is hawkweed taking over some newly sunny ground

    With the idea that this could be a good, I’ve been looking up our weeds in Wild Urban Plants of the Northeast, Peter Del Tredici’s guide to “spontaneous urban vegetation.” Many are listed as “disturbance-adapted colonizer of bare ground,” true to their behavior in my yard.

    One of their useful functions is to provide food and habitat for beneficial insects, the ones that prey on leaf-eaters. Although these insects are predators in at least one life stage, they often need nectar and pollen in another. That’s one way that weeds such as lamb’s quarters and chickweed can be a plus.

Lamb's quarters wilting in this week's heat

Their flowers provide extra forage and fill in during the lean times when our preferred flowers aren’t blooming. Weeds may also attract prey insects that the beneficials need to eat or parasitize. 

    Here are some of the weeds that have popped up where the hemlocks grew:

Pokeweed (Phytolacca americana): This is a North American native that, although toxic to mammals, has fruits that birds love. At least 30 bird species use pokeberries as a major food source. 

Pokeweed grows quickly into a substantial presence

Warblers and other migratory birds eat them on their flights south. The berries help mockingbirds, cardinals, cedar waxwings, and others to get through the Northeastern winter. Pokeweed is larval food for several moth and butterfly species, including the giant leopard moth. Bees feed on its pollen.

Giant leopard moth-photo NPS Phillip Brown

Yellow woodsorrel (Oxalis stricta): Also a native, this low perennial shows up all over my garden. At summer camp we called it sour grass and chewed on the leaves when we were thirsty on a hike. 

Recognize this yellow woodsorrel? It's all over the place.

The plant’s tissues are high in oxalic acid, so it’s better to sample the leaves in moderation. It too provides seeds for birds, pollen for native insects, and leaves for butterfly and moth caterpillars.

Devil’s beggarticks (Bidens frondosa): This native annual gets the prize for its many colorful names: sticktight, devil's bootjack, pitchfork weed, Spanish needles. You can see it hasn’t made a lot of friends among gardeners, but it provides pollen for bees and, interestingly, food for muskrats when it grows near water.

Devil's beggarticks has pretty flowers; its bad rep comes from the burrs that follow.

    These weeds are easy for me to pull out, especially because they’re growing where I previously put down a layer of wood chip mulch. Under the wood chips, the soil stays soft. I’ll remove some weeds that are shading my squash and melon plants (a story for another day), but now I know to leave some for their contribution to the garden’s ecology.