Spring is in the air, little green things are popping up all over, and we all heave a sigh of relief that the blanket of white stuff is finally gone. But beneath the snow that stopped everything in its tracks lurks a hardy, robust little puff of tiny green leaves that virtually grows before your eyes.
(This article was originally published on March 29, 2010. Your comments are welcome but please be aware that authors of previously published articles may not be able to respond to your questions or comments.)
Call it what you will – hairy bittercress, winter bittercress, hairy cress, popping cress – Cardamine hirsuta – is a weed that tries the most forgiving gardener’s patience. Growing worldwide (except in the Antarctic, this genus of the Brassicaceae family numbers more than 150 species, both annual and perennial. The plant is self-pollinating and in bloom throughout the year. It loves moist soil and grows aggressively under those conditions.
As the snow melts, tiny white, pink, or lavender flowers begin to appear. Yes, flowers. This tenacious weed is short-lived, which is good, you say. A life cycle of 6 weeks doesn’t seem like such a big deal. Think again – how many 6-week cycles are there in a year?
One of the biggest problems with bittercress is that, by the time you discover you have a problem, it’s almost too late to do anything about it. The first flowers appear in late February or early March, quickly form seed pods, and mature. If you touch those trigger-happy seed pods, i t’s all over – the pods explode, distributing seeds over an area up to 36 inches around each plant. Those seeds will germinate and begin sprouting with a few days and the cycle begins again, only over a larger area. Small to medium size plants produce about 600 seeds, and larger plants can yield up to 1,000 seeds.
Hairy bittercress is not invasive enough to warrant using herbicides. As soon as new plants appear in February or March, begin pulling them; these are the offspring of the previous fall’s seed crop. Through the season, always pull the seedlings when you see them; they have shallow roots and come away quite easily; however, bits of root left behind are capable of re-rooting under optimum conditions. The key is to get the plants before they set seed, which happens quickly after blooming. Eradicating this weed from large areas is almost impossible, unless you can hoe and remove. Keeping bittercress out of the flower beds is a little easier, but requires diligent hand-weeding to stay ahead of the seed formation. The leaves release a pungent aroma when bruised.
Hairy bittercress is a problem in greenhouses and nurseries, so be sure to clear off the top 2 to 3 inches of soil before planting anything you purchase. Scoop the soil into a plastic bag and dis card. Keep a close watch on newly planted containers, especially those that are positioned near flower beds. The propulsion factor of bittercress seeds can sneak new plants into your containers while you aren’t looking. Hairy bittercress is a real problem near flagstone patios or walks, brick work, or any hard-scaping that has space between the pieces. This weed does not need much to set down roots – even a small amount of sand between two bricks is plenty.
While this plant can be edible, it is also a very invasive weed here in the Missouri Ozarks where I live. I spent 2-1/2 hours pulling it one day.
Though hairy bittercress is originally from Eurasia and was introduced to North America, there are several species of Cardamine that are native to the United States. Several are listed as threatened or endangered, mostly due to habitat loss.
As winter warms to spring, a favorite weed of foragers starts to emerge in rather cute clumps—it’s hairy bittercress! It has actually been lurking near the surface all winter, having germinated in the fall and waited out the cold temperatures before sending up flowers and seeds.
Plan Your Visit
Hairy bittercress is adapted to moist, disturbed soils, so it emerges wherever we irrigate. Unsurprisingly, then, it’s a common lawn weed (where it can form expansive mats) as well as a greenhouse weed (where it pops up in and around containers). Mowing and hand weeding are the typical means of control—the shallow fibrous roots make it an easy pull. If you do pull some from your garden beds, consider making a farmer’s sandwich of cheese, apples, and a bit of fresh bittercress. Skip the compost pile and send it your stomach instead!
More: Learn to identify more weeds and find out more about each one by browsing the Weed of the Month archive.
The Weed of the Month series explores the ecology and history of the common wild plants that most gardeners consider weeds.
While urban grazers will be most focused on the leaves, I think the seed capsules are the best part of hairy bittercress. Called siliques, they look like purplish-green toothpicks standing upright around the flower. As the seeds mature, the pods begin to coil tightly until—pop! A gentle touch or passing breeze triggers the pods to explode and send the seeds flying as far as three feet from the mother plant. This ballistic dispersal strategy, known as ballochory, is also employed by jewelweed and cranesbill.
One of the largest groups of plants that uses ballistichory is the pea family, or Fabaceae. This is just one type of plant that shoots seeds when touched and the pod is cracked open. Lupins (Lupinus spp.), a garden favorite that’s hardy in U.S. Department of Agriculture plant hardiness zones 3 through 9, form columns of pea-like fruits that burst open when dry. Orchid trees (Bauhinia spp.), hardy in USDA zones 9 through 11, bear large pods that can fling seeds nearly 50 feet. Gorse (Ulex spp.), an aggressive broom-type plant that is considered a noxious weed in some states, makes a popping noise when the seed pods burst open.
Some plants disperse their seeds forcefully by ejecting them. Sometimes the tension is so great, seeds may be ejected up to 200 feet away from the mother plant. This method of seed dispersal is called “ballistichory,” a label that hints at the projectile-like emergence of seeds from their pods or capsules. This type of seed dispersal occurs because the fibers in the dried fruit pull against each other to create tension, and when the tension is great enough, the fruit splits open and the walls of the fruit spring back, flinging the seeds out with force.
Plants in the Fabaceae Family
Another plant that shoots seeds when touched is Euphorbiaceae. The Euphorbia family produces segmented seed capsules that somewhat resemble a peeled orange in shape. Examples of plants in this family include bitterbark (Petalostigma spp.), the sandbox tree (Hura crepitans), and the rubber tree (Hevea spp.)
The Acanthus family (Acanthaceae) produces a number of shrubs that distribute seeds through explosive action. Most produce highly ornamental flowers. The Mexican petunia (Ruellia spp.) has small petunia-shaped flowers and produces pods that disperse seeds up to 10 feet away. Mexican petunias are hardy in USDA zones 8 through 10, and may be an invasive plant in some states. Justicia produces colorful flower spikes and is hardy in USDA zones 9 through 11. The seed capsules explode when mature. Firecracker plants (Crossandra spp.) are aptly named because of their pretty orange flowers and exploding seed pods. These capsules dry out first then pop open once they become moist again.
Bitterbark is a native of Australia and hardy in USDA zones 9 through 11. The dried fruits explode and send seeds three feet into the air. The sandbox tree sends its seeds hurling nearly 150 feet, and the exploding seed pods create a loud noise comparable to a rifle discharge. The sandbox tree is hardy only in USDA zones 10 through 11. The rubber tree , the source of natural rubber, can shoot its seeds 50 feet from the tree. It’s hardy in USDA zone 10 through 11.