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5 of the worst garden weeds and what you can (or can’t) do about them

October 7, 2023

“Fall is in the air. Time to plan spring blooms.”

These words of horticultural wisdom come from Jenna Christensen who gardens in Manhattan Beach. New beginnings in the garden come twice a year: In the fall, when we plan and plant according to what we want to bloom next year and beyond, and in the spring when whatever we plant now puts on a spurt of growth and bulbs, at least, sprout glowing, smile-producing flowers that make the winter wait for them worthwhile.

According to Jewish tradition, the world was created in the fall and Adam and Eve were created on Rosh Hashanah, the first day of the Jewish year. The idea of fall creation makes sense since the first couple had an immediate source of food in the form of tree fruit – much of which ripens in the fall – ready to be picked. And there was lots of fruit to choose from as God tells Adam: “Of every tree of the garden you may freely eat, but of the Tree of Knowledge of good and evil you shall not eat” (Genesis 2:16-17). Of course, Adam and Eve could not resist the forbidden fruit and, as they say, the rest is history.

But what were the horticultural consequences of this regretful act? Weeds! To quote the Biblical account: “Because you ate from the tree I commanded you saying: ‘You shall not eat of it,’ cursed be the ground … it will grow thorns and thistles for you” (Genesis 3:17-18). Indeed, the curse of the garden is weeds and we can only dream of what life would be like without them.

Here is a list of five of our most pernicious weeds:

1. Bermuda grass (Cynodon dactylon)

As a lawn grass, Bermuda is desirable for its drought tolerance. In order to thrive, it needs regular water in hot weather, if not daily irrigation, but it can survive virtually without water owing to its triple insurance policy: underground rhizomes for long-term energy storage in the form of starch; above-ground stolons or runners that root wherever a node touches the soil surface; and deep roots, which may go down as far as 10 feet. In an ornamental or vegetable garden, hand-pulling will keep Bermuda grass under reasonable control and four inches of mulch above a layer of newspaper will have a depressing effect on its growth.

2. Field bindweed (Convolvulus arvensis)

Convolvulus is a wiry-rooted weed with attractive white or pinkish-white gramophone-shaped flowers. It’s called bindweed because it binds itself and winds itself around the stems and leaves of everything in its path and, if nothing is in its path, it winds itself around itself. It can never be completely dug out owing to its Bermuda-like rhizomes and its taproot that defies deracination. In addition, its seeds remain viable for more than 20 years.

To control bindweed in the garden, don rubber gloves and spray a systemic herbicide onto a sponge. Sponge the leaves and shoots of your bindweed and watch it wither away. If you are adamant about the avoidance of toxic chemicals in the garden, you can kill it and most other weeds, for that matter, with a 20% vinegar solution (4 parts water: 1 part vinegar). The problem is that whatever vinegar touches it will kill so you have to exercise caution in targeting weeds while directing spray away from desirable plants.

3. Wood sorrel or oxalis (Oxalis corniculata)

Wood sorrel is another attractive weed. Novice gardeners often mistake it for clover, because of its shamrock foliage. It also has the look of an ornamental ground cover, due to its mounding growth habit and attractive, butter-yellow flowers. There are two commonly seen types, one with green and one with maroon to deep violet-colored leaves.

The problem with oxalis eradication is its wiry tap root and explosive seed capsules. If you scrape or hoe it off to ground level, it will simply grow back. If you try to dig out its roots, you will be at great pains to remove them completely because they grow in a web, easily break apart and defy smooth extraction.

You may decide that, well, this plant is actually kind of attractive, so why not just let it take over the flower bed? You may even excuse your inactivity by recalling that famous maxim of weed scientists, namely that “a weed is a plant for which no useful purpose has yet been found.” The problem with oxalis is that it does not stay confined to a single flower bed, but shoots its seeds six feet in every direction so that it will soon become a garden-wide headache. Control it like you would field bindweed.

4. Black mustard (Brassica nigra)

This is the most widely distributed weed in California. It’s a winter annual which means it dies in summer but comes up with winter rain, flowering in spring and summer. It is actually quite a spectacle to see a mass of it in bloom with its heavy load of yellow flowers. Control it through solarization which involves soaking it when actively growing, after which it is immediately covered with clear plastic, after which it dies in the steam heat that is generated and trapped under the plastic when the summer sun beats down upon it.

5. Nutsedge or nutgrass (Cyperus esculentus)

This is widely considered to be the worst weed in California. It is easily identifiable by its shiny leaf blades and hard, nutlike underground tubers. Complete eradication may not be possible. But there are some anti-nutsedge chemical products, available in garden centers and through the Internet, that you might want to try. If you are opposed to chemical use, you will probably have to sell your house and move to another, after carefully inspecting the garden of your home-to-be to make sure no nutsedge is present. Still, heavy mulching, as in a six-inch layer of wood chips, may be effective for nutsedge control if you can keep that thickness of mulch present at all times.

There are relatives of nutsedge that are more garden-friendly. Umbrella plant (Cyperus alternifolius) grows up to 5 feet tall with many parasol-shaped leaves. It is valued, in some quarters, for its durability as a container plant, whether on the patio or indoors. The problem with umbrella plant is that it, too, may become weedy. However, if you begin to see too much of it, you can eliminate it through simple excavation.

The most famous nutsedge relative is papyrus (Cyperus papyrus), encountered both as an aquatic and partial-shade garden specimen, growing to 6 or 7 feet tall. Misled, perhaps, by its somewhat wispy and delicate-appearing foliage, some people make the mistake of giving papyrus too much shade, which will inhibit its growth or kill it outright. Make sure that papyrus has good ambient light, but take note that ‘King Tut,’ a 2-to-3-foot-tall dwarf papyrus, is a bit more shade tolerant.

California native of the week: Blue-eyed grass (Sisrynchium bellum) is actually a perennial member of the iris family although its foliage gives it a grassy look. It’s one of the few natives that prefers heavy soil and you should probably not have to be concerned with over-watering it. Its half-inch, star-shaped blooms come in the spring and the plant may go completely dormant in summer. It may grow up to two feet high and wide although there are dwarf cultivars that stay beneath a foot tall, one of which is available at Artemisia Nursery (artemisanursery.com) in the El Sereno area of East Los Angeles, which is described on the nursery’s website as “good under oaks, full sun to part sun along the coast to part shade inland.” The San Simeon cultivar grows to only four inches tall with white flowers, while golden-eyed grass (Sisrynchium californicum) also grows up to two feet tall with shiny yellow flowers and a strong capacity for self-sowing. Most native plant nurseries should carry blue-eyed grass and the Theodore Payne nursery (theodorepayne.org) has its seeds available as well.

I am seeking a way to keep nocturnal visitors – raccoons, skunks, and rodents – from chewing on my irrigation lines. I have made countless repairs to these lines due to animal damage. This is the first year I have experienced this problem and I am wondering if the heavy rains led to the proliferation of urban wildlife in search of water.

In any case, if anyone has experienced this problem and found a solution to it, please share your success in an email to [email protected]. Your questions, comments, and descriptions of garden problems or pests are always welcome, too. If you have a plant in your garden that more of us should know about, please take a photo of it with horizontal orientation and send it to this same email address for possible publication.

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5 of the worst garden weeds and what you can (or can’t) do about them

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