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Hairy bittercress is an annual weed that can spread quickly. Official Blog of the UC Master Gardener Program of Contra Costa County There?s a hitchhiker lurking in your car right now and you don?t even know it. Fortunately, it?s not the kind that might end up on the nightly news, but it?s almost as bad where the ecosystem is concerned. Learn more about hitchhiking weeds in this article.

Hairy bittercress: A weed to watch out for

Hairy bittercress is an annual weed that can spread quickly.

Flowers and seed pods of hairy bittercress. Photo by Lori Imboden, MSU Extension.

Have you recently noticed plants with small, white flowers on the edges of your lawn, flowerbeds and rock pathways? During April and May, populations of the winter annual weed hairy bittercress (Cardamine hirsuta) become increasingly visible. Hairy bittercress has a low growing rosette similar in form to a dandelion. It raises its profile in early spring with the appearance of flowers and seeds on a vertical stem. Like many members of the mustard family, hairy bittercress sets seed prolifically. It grows quickly and a few plants or seeds can generate a more widespread infestation in even a year’s time.

The first true leaves of hairy bittercress are heart shaped. Photo by Erin Hill, MSU.

Hairy bittercress is a winter annual weed. Its seeds germinate in fall beginning as early as September. The first true leaves are heart-shaped, followed by compound leaves with two or more pairs of leaflets and a kidney shaped terminal leaflet. The leaves that emerge in the fall form a small rosette that will overwinter. Once the weather warms in spring, it sends up stalks of small, white flowers followed by slender seed pods known as siliques.

Hairy bittercress leaves have two or more pairs of leaflets and a kidney shaped terminal leaflet. Photo by Lori Imboden, MSU Extension.

Once the seed pods ripen, disturbing the pods can propel the seeds as far as 16 feet from the mother plant. This seed dispersal adds to the soil seed bank and primes the area for another infestation to emerge in early fall. After setting seed, the life cycle is complete and the plants die. Hairy bittercress and other winter annual weed species, like common chickweed and purple deadnettle, are not typically present during the summer months.

Once the seed pods ripen, disturbing the pods will send the seeds flying as far as 16 feet. Photo by Lori Imboden, MSU Extension.

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Hairy bittercress is best managed mechanically when it is young. Remove it by hand, hoe or tillage in early fall or early spring before it sets seed. If plants are flowering, composting is discouraged as seeds may develop. To manage this weed using herbicides, the proactive approach would be to use a pre-emergence herbicide in the late summer (late August to early September) to target the plants at the time of germination and prevent successful emergence.

If plants have already emerged, applying a post-emergence herbicide to actively growing plants before seedpods form may be effective. If using an herbicide, be certain it contains an active ingredient that will target this weed. Always read and follow all labeled instructions to increase effectiveness and prevent personal or environmental harm.

Seed Pod Weed

Hairy Bittercress (Cardamine hirsuta)

I’ve become fascinated with this weed, at least so far. I’ve more-or-less got it under control in my garden. I really don’t remember it from years ago, but it sure has been a pest the last 5 years or so. Not a native of California, it is now here for the foreseeable future… and beyond. I can’t say it’s the worst weed in the garden, but it sure requires attention to keep it under control. Especially these days when it will be competing for available water.

Even if you didn’t recognize it outright, maybe you’ve had the experience of being out in the garden pulling winter weeds when you’re pulling what looks like a small “innocent” weed only to find it exploding seeds in your face and all over the nearby garden? The most likely culprit is Hairy Bittercress (Cardamine hirsuta), which starts growing almost immediately with the onset of the late winter rains. It originally looks like a small, flat rosette of small leaves and then in what seems like the next day, quickly produces small white, four-petal flowers on wiry green stems. Most of the problems with this weed can be “solved” if you pull it at this stage or at least before it flowers. Seemingly overnight, the flowers form needle-thin seed pods, which explode at the slightest touch, sending seeds in all directions (averaging around 600 seeds per plant… and the bigger the plant, even more seeds). Besides your garden, the seeds are easily propagated in cracks in flagstone, brick or concrete walkways.

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Because Hairy Bittercress thrives in moist conditions and disturbed soil, it is also a pest in nurseries, and can be brought home via plant purchases. If you think that’s a problem for you, some cautious gardeners carefully remove the top inch or two of soil in the pots before planting. (If you do this, you should dispose of the scraped-off soil in your green can.)

If all else fails, Hairy Bittercress is a member of the mustard family and is edible, but you need to do your own research to find the right recipe to enjoy it (for an example, see http://www.eatingniagara.com/2013/04/weed-wednesday-make-that-hairy.html). To get ahead of its persistence in the garden, it’s definitely worth patrolling your garden for this weed once or twice a week during the winter and spring. It’s easy to hand pull when young. Once the seeds pop, you’ll be fighting a much bigger crop next year and it’s rare that herbicides would be considered appropriate for control in a home garden.

Another reason it’s my “favorite” weed? I still remember a fellow student in our Master Gardener class relating how she had “convinced” her young son to help weed the spring garden and he was complaining about the weed seeds popping in his face. She answered him by telling him to go in the house and get his safety goggles on and keep on weeding… something you might be considering adding to your gardening tools if you let Hairy Bittercress (Cardamine hirsuta) go to flower and reseed.

Seeds That Stick To Clothing: Different Types Of Hitchhiker Plants

Even now, they’re lingering along the roadside waiting for you to pick them up and take them wherever you’re going. Some will ride inside your car, others on the chassis, and a few lucky ones will find their way into your clothing. Yes, weeds that spread by people, or hitchhiking, have certainly taken advantage of you this year. In fact, the average car carries two to four seeds for hitchhiker plants at any given time!

What are Hitchhiker Weeds?

Weed seeds spread in a variety of ways, whether traveling by water, air, or on animals. The group of weeds nicknamed the “hitchhikers” are seeds that stick to clothing and fur, making it difficult to dislodge them immediately. Their variously barbed adaptations ensure that the seeds will travel far and wide via animal locomotion, and most can be eventually shaken off down the road somewhere.

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Although it might sound like all fun and games, the weeds spread by people are not only difficult to contain, but they’re also costly for everyone. Farmers lose an estimated $7.4 billion each year in productivity to eradicate these pest plants. Humans are spreading these seeds at a rate of 500 million to one billion seeds a year in cars alone!

Although the weeds within crop stands are annoying, those that appear in fields can be downright dangerous for grazing animals like horses and cattle.

Types of Hitchhiker Plants

There are at least 600 weed species that travel by hitchhiking with humans or on machines, 248 of which are considered noxious or invasive plants in North America. They come from every kind of plant, from herbaceous annuals to woody shrubs, and occupy every corner of the world. A few plants you might be familiar with include the following:

  • “Stick-tight” Harpagonella (Harpagonella palmeri)
  • “Beggerticks” (Bidens) (Krameria grayi) (Tribulus terrestris) (Opuntia bigelovii) (Torilis arvensis) (Symphyotrichum lateriflorum) (Arctium minus) (Cynoglossum officinale) (Cenchrus)

You can help slow the spread of these hitchhikers by carefully inspecting your clothing and pets before emerging from a wild area full of seeding plants, making sure to leave those unwanted weeds behind. Also, reseeding disturbed areas like your garden plot with a cover crop can ensure that there’s too much competition for hitchhikers to thrive.

Once those weeds emerge, digging them out is the only cure. Make sure to get 3 to 4 inches (8-10 cm.) of root when the plant is young, or else it’ll grow back from root fragments. If your problem plant is already flowering or going to seed, you can clip it at the ground and carefully bag it for disposal – composting will not destroy many of these types of weeds.

Last, but not least, check your car any time you’ve been driving on unpaved roads or through muddy areas. Even if you don’t see any weed seeds, it wouldn’t hurt to clean your wheel wells, undercarriage, and any other location where seeds might be hitching a ride.

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