One Gardener’s Weed… or Cover Crop?

I’ve been thinking about weeds and wildflowers a lot this week, probably because I spotted my first field bindweed in a gravel path. But it’s also because I’m thinking about spring flowers, pollinators and soil.

alyssum mustard weed wildflower
Last year’s “crop” of yellow flowers. Pretty but a little scary.

In past posts, I’ve described the stages of “acceptance” we’ve gone through with the yellow flowers that pretty much blankets our property. In brief: The plants are similar (and related) to wild alyssum. Technically, they are a type of Physaria, also called twinpod or bladderpod. The plant is good because it attracts bees – hundreds of them. And it’s pretty, creating a sort of spring meadow. But it’s bad too. With a deep tap root, the weed/wildflower, (a member of the Brassica, or mustard, family) likely steals water and some nutrients from the grass or garden plants.

Following an excellent Twitter chat on nitrogen fixers (#groundchat led by @CristinaGardens; also on Facebook), I decided to check again whether this mustard/alyssum helps or hurts soil. It turns out that yellow alyssum does not fix nitrogen, but has a role in recycling the nutrient. It can help smother weeds, but then is it smothering or delaying grass? Yes, it threatens native grasses, according to the Colorado State extension office. And in a xeric garden, I don’t want a plant that robs vegetables or perennials of precious water.

sweet alyssum new mexico
This Physaria against an old tree trunk looks like we planted it intentionally.

Then I found an article saying that the long tap roots on plants help till or break up soil with their long taproots. They also can bring nitrogen back up to the surface, or at least help conserve it in soil. Shouldn’t that be good for grass? And many varieties of Phyaria are endangered. Not so much in our neighborhood…

When the roots decompose, they add organic matter. So, I’m back to accepting alyssum/physarium for these bonus qualities, but mostly because we’ll never win.

green beans on vine
Green beans add a little nitrogen, but most of the nutrient goes into producing fruit. And that’s OK with me.

Let’s go back to the nitrogen fixers for a minute. Plants need nitrogen to survive. How much a given plant needs varies, but it’s safe to say that nitrogen is an essential plant nutrient. Many gardeners who use fertilizers know this; the N in formulas represents nitrogen. But why apply a synthetic fertilizer to crops or a garden when you can harvest nitrogen with plants that do the work? It’s the organic approach to soil improvement that’s really catching on with farmers. Hurrah!

red clover blossom
Bloom of a red clover. Image courtesy of the National Park Service.

Examples of nitrogen fixers are hairy vetch, annual wheat grass or ryegrass, and legumes, including clover. Wait, what? I’ve always considered clover invasive. And during the chat, I received an excellent link to an article about sweet clover. Looks like clover is to many gardeners and farmers what the mustard family is to us – good or bad, invasive or not. It all depends on your conditions. Further, conditions that favor a particular weed or wildflower can change slightly from one year to the next. Red clover is a potential nitrogen fixer in New Mexico.

sweet peas near river
Sweet pea is a legume and natural nitrogen fixer. It also can become invasive, but it’s a pretty surprise growing along the Rio Ruidoso.

And that’s why it’s all relative. I love learning from social media, and learned a lot the other day. But when choosing plants, especially for a large area such as a meadow or field, it’s always best to check with local extension agents or their publications. Otherwise, gardeners in New Mexico would never plant butterfly bush (Buddleia), which is not invasive here, and a gorgeous xeric choice. And they might go a little wild with mustard natives or horehound, which are terrific additions to gardens in some areas, but can take over a New Mexico lawn.