After yesterday's lovely UCMGMB Garden Tour and all the well-loved and well cared-for gardens we were so lucky to visit, today's post is about the gardens, roadsides, ag fields, and abandoned lots of our area that still need some green thumb action. While the worst of the spring flush of weeds may now be past, hopefully this post will prepare us all for next year so that we can attack these weeds at the beginning of the season and eradicate them!

In my UCCE Master Gardener class we defined a weed as any plant that grows where you don't want it to grow. So that means it's kind of subjective. But there is general agreement among many gardeners, agriculture folks and sustainability folks about the identification of some common weeds in our area. I'll talk a bit about each one, give the common and scientific name, how to get rid of it, and direct you to any pest notes or other resources for more information. All of the “weeds” I'll be discussing today are not native to the Central California area and in fact, are not native to this country.

Bermuda buttercup or sourgrass (Oxalis pes caprae)

Yellow oxalis blooms are pretty. Unfortunately oxalis is a pretty nasty character, able to create monocultures and crowd out other species. Surprisingly, oxalis doesn't reproduce sexually, but only through the production of bulbs on the roots. When you pull up an oxalis plant, those bulbs often break off, remaining in the soil to produce more oxalis the next year. The most effective way to get rid of it is also the most time-consuming: dig it up and carefully sift through the soil to remove all the bulbs. You can also pull it up every year before it flowers, which will eventually use up the bulb's carbohydrate resources. But you'll have to do it for several years in a row and if you miss some and it flowers, beware! After flowering it has already produced new bulbs that will produce plants the following season. Alarmingly, this stuff has invaded our coastal dunes and started crowding out native dune plants. It's edible, but high in oxalic acid, so be careful if you decide to add it to your salads.

IPM Oxalis Pest Note

Cal-IPC Oxalis Plant Profile

 

Black mustard (Brassica nigra)

I hate to break it to the people out there who rejoice in the fields of yellow flowers, but this is a weed. While some songbirds may eat the seeds and some farmers use it as a cover crop, there are better alternatives for habitat and agriculture out there. And there's a good reason why you often see black mustard spreading to fill an area: it produces chemicals that prevent the germination of native plants. As it spreads, black mustard can increase the frequency of fires in chaparral and coastal sage scrub, changing these habitats to annual grassland. Hand pulling after a rain is probably the easiest way to eradicate this plant. It has a sturdy taproot, so if it breaks off it may resprout. You can also cut plants below the root crown with a pick or shovel. Any removal strategies should happen before seed pods develop.

Cal-IPC Mustard Plant Profile

IPM Mustards

 

Wild radish (Raphanus sativus or R. raphanistrum)

This is another species that people sometimes find pretty. I know, I know, the little pink, white, or lavender flowers are kind of pretty. And some folks like it because honeybees and other insects forage on it. But this is actually the same species as the radish we grow for consumption and it can hybridize with crop radishes, introducing weedy characteristics into the crops. In addition, it can host pests such as cucumber beetles, flea beetles, and stinkbugs that can cause crop losses. Radish-mosaic virus is a common disease in cultivated radishes from gardens in our area, and the disease also occurs in wild radish, which probably serves as a reservoir of the virus. Hand pulling is the easiest removal method for wild radish, but again, it has a big taproot, so after a rain may be easiest. If the taproot breaks it may resprout. Cutting plants below the root crown with a pick or shovel also works. For large areas, repeating mowing may be necessary to eliminate both resprouting older plants and newly sprouted seeds. And try to do all removal before the seed pods develop.

For some pro-wild radish info:

Bug Squad - Wild about Radish

For some anti-wild radish info:

From the California Agriculture journal, an article entitled "When crop transgenes wander in California, should we worry?"

UCANR Green Blog entitled "Hedgerows enhance beneficial insects on farms in California's Central Valley"

 

Poison hemlock (Conium maculatum)

Poison hemlock is also blooming in some locations right now, although in my yard it is just starting to poke up its lacy fronds that kind of remind me of carrot greens. It's called “poison” hemlock for a reason. The sap of the plant can cause blistering on your skin and if too much is ingested it can kill livestock. Poison hemlock can also crowd out other, more desirable plants. Hand pulling, mowing, biological and chemical controls are all available. Best is to prevent a small-scale infestation before it spreads. 

IPM Poison Hemlock Pest Note

From the UC Master Gardener Program Advice to Grow By blog, a post entitled "Poison hemlock widespread in California"

 

Italian thistle (Carduus pycnocephalus)

While Italian thistle may not be as big a problem as yellow starthistle, it can still be very hard to eradicate when it gets into your yard. There is a biological control agent, the Thistle-head Weevil, which was introduced to Northern California, where it lays eggs on Italian thistle flower clusters and then larvae burrow into the forming seed heads and eat the immature seeds. Unfortunately, the weevil also attacks many native thistles. Italian thistle produces mucilaginous seeds that can travel more than 325 ft in strong winds, so it easily dominates sites and excludes native species. A single plant can produce 20,000 seeds in one season! Hand pulling is probably the best control method, but you have to make sure you sever the root at least 4” below the ground or it may regrow. Mowing is not reliable because the plant can still set seed, even after repeated mowings to 3” high.

Cal-IPC Italian Thistle Plant Profile

From the UCANR Hopland Research and Extension Center's blog post "Weed biological control at work at Hopland"

 If you've got any questions about these or other weed species, feel free to call our Master Gardener Hotline at 831-763-8007 or get in touch online at our website!