Why ‘Good Weed’ Pennycress Isn’t As Good For Your Lawn As You Think

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Why 'Good Weed' Pennycress Isn't As Good For Your Lawn As You Think

Alpine pennycress is a small weed that has been gaining attention for its ability to remove toxic heavy metals from soil. There is a lot of research supporting this, and it shows promise for cleaning up contaminated sites. However, it’s important to note that this plant may not be suitable for your lawn. One of the most significant factors to consider is that it may only thrive in certain countries.

When you come across claims about plants’ environmental purifying qualities, it’s essential to take them with a grain of salt. Even research that’s based on solid evidence can be misleading if there isn’t enough context. For instance, NASA conducted a study that showed some potted plants could clean the air in your home, but it didn’t prove this. While some houseplants can purify the air to some extent, it’s a complex issue. But some plants, like alpine pennycress, can live up to their rumored potential.

Alpine pennycress, scientifically known as Thlaspi caerulescens, is a hyperaccumulator of heavy metals, meaning it can store at least 100 times as much as ordinary plants. It can accumulate heavy metals such as arsenic, lead, cadmium, and zinc at high levels. This is a good thing, but it’s important to consider whether it’s suitable for your lawn.

There are two alpine pennycresses

Thlaspi montanum aka alpine pennycress

Matt Lavin/CC BY-SA 2.0/Wikimedia Commons

If you’re thinking that a field of alpine pennycress is just what the doctor ordered to clean up your abandoned mineshaft, there are a few gotchas. First and most importantly, the research indicates that while the variety of pennycress that’s native to southern France (T. caerulescens) can clean up several heavy metals, the variety that grows in the U.S. (T. montanum) only removes significant amounts of nickel. And that’s good because high levels of nickel can be disastrous for plant populations, most of which can’t tolerate it. It’s bad for people, as well. The trouble is that both of these varieties are called alpine pennycress, which has led some to mistakenly claim the alpine pennycress at the edge of your lot can do the things only the European alpine pennycress can actually do.

Scientists and master gardeners use binomial nomenclature –- those unpronounceable two-word scientific names –- for plants, bugs, and other living things just to avoid problems like this. And even that system is fraught with problems. In fact, T. caerulescens has at least 16 different scientific names. But at least we can be reasonably sure T. caerulescens is one plant and T. montanum is another entirely, an advantage we don’t get from calling both “alpine pennycress.”

Other reasons to pause your pennycress farm

French variety of alpine pennycress

Joan Simon/CC BY-SA 2.0/Wikimedia Commons

The other problem is that alpine pennycress is a small plant that doesn’t have a lot of biomass, so it can’t store enough heavy metals to clean up a really toxic area. And even if you did grow it, you’d need some means of harvesting it efficiently to reap its benefits; otherwise, some of the heavy metals would just be returned to the soil as the annual decays. It would be nice to reduce heavy metals in your property’s soil, which can find its way there in several ways, including via common items you might use in your yard. But at the moment, this property of alpine pennycress is mostly used by researchers to investigate new ways to deal with contamination.

Interestingly, T. montanum, the alpine pennycress that grows in the western U.S., turns out not to be from the U.S. at all. It’s an invasive, though not entirely objectionable, transplant from Europe. So it’s possible that its French cousin could also make its way here and set up shop. And maybe all this research will allow T. caerulescens to be permitted as an import for planting (as a cover crop, for example). Then, all those articles proclaiming the benefits of alpine pennycress will suddenly be true … assuming you happen to have the right one. Please note that while some fans report that pennycress is edible, this might be inadvisable given the plant’s habit of storing high levels of heavy metals.

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Dr Heidi Parkes

By Dr Heidi Parkes

Senior Information Extension Officer QLD Dept of Agriculture & Fisheries.