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weeds that throw seeds when touched

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.

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.)

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.

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.

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.

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.)

Acanthaceae: Exploding Shrubs

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.

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.

Some plants in the mallow family (Malvaceae) disperse seeds via ballistichory. The common garden vegetable okra (Abelmoschus spp.) is a plant that shoots seeds when touched, projecting its seeds several feet. Okras are annuals that are hardy in USDA zones 4 through 11. The kapok tree (Ceiba pentandra, also known as cotton tree) is native to South America and produces cream-colored flowers with an unpleasant smell. The pods burst open, sending both seeds and the exploding seed pod’s cottony insulation into the wind. Kapok trees are hardy in USDA zones 10 through 11.

If you scalp the lawn, weeds will thrive. If you water it frequently for short periods of time, weeds will thrive. And if you feed the poor heat-stressed thing in summer, weeds will take over.

If I’m paying attention and life cooperates, I’ll pull the weeds while they’re still in flower and before they set seed. Both weeds get composted—mixed into a good amount of shredded leaves hoarded from the previous fall; at least two parts leaves to every part green weed. The bitter cress typically comes up with a good amount of soil attached to its roots, which adds microbial life to the pile; and the chickweed has a lot of water content to help keep the moistness levels right.

If I don’t get to them in time, I toast the seedheads with my trusty flame weeder before I pull the plants, just like I do with dandelions that have progressed to the puffball stage. Dandelion seeds burst into little flares of color—like Munchkin fireworks. Bittercress seeds explode with a loud ‘pop’. (Organic gardening is SO much more fun than spraying hormonal disruptor around!)

Its small white flowers are similar to those of chickweed, another ‘unwanted plant’ that blooms early in the Spring. But chickweed is more of a flat, spreading, mat-like plant. And its seedpods aren’t faster than a speeding bullet. Both weeds are remarkably easy to control in flowerbeds; just pull them, roots and all, out of wet soil. Chickweed comes out in big clumps, while bitter cress has a nice little stalk that gives you a handle to grab onto. Just remember to soak the soil first; all weeds come out of wet soil MUCH easier than dry.

In turf, weeds like bittercress are a sure sign of poor lawn care. The answer is not to poison yourself and the environment (and kill your grass) in a futile attempt to remove the weed, but to care for your lawn correctly and deny the weed a place to live. Take good care of your grass and a harmless little plant like this should never have a chance to get established, much less thrive.

Both weeds are also highly edible, especially when young. Chickweed is more nutritious than the salad greens that many people remove it to plant! And, although hairy bittercress (a member of the mustard family) doesn’t have nearly as many wild food fans as chickweed or purslane (perhaps the most edible ‘weed’), it does have some of the peppery taste of its namesake watercress, and it’s loaded with cancer-fighting nutrients. Pick it before the flower buds form and it won’t have nearly as much of the bitter edge that older plants take on. (Flowering changes the flavor of virtually all herbs and greens for the worse.)

But I like to wait until after the little white flowers form to pull these weeds. Their flowers open up right before the blooms on my fruit trees, attracting lots of the pollinators and beneficial insects I’ll need to get a good fruit set and to fight all the pests that want to eat those peaches as much as we do.