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

In past posts, I’ve described the stages of “acceptance” we’ve gone through with the yellow alyssum that pretty much blankets our property. In brief: The alyssum 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 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 alyssum against an old tree trunk looks like we planted it intentionally.

Then I found an article saying that the long tap roots on our yellow alyssum (and presumably mustard weed) help till or break up soil with their long taproots. They also bring nitrogen back up to the surface. Shouldn’t that be good for grass? Alyssum helps in some areas when planted as a cover crop following a nitrogen-rich legume.

Further, when the roots decompose, they add organic matter. So, I’m back to accepting alyssum 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 sweet alyssum 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 the sweet alyssum or horehound, which are terrific additions to gardens in some areas, but can take over a New Mexico lawn.