TIME TO MAKE PEACE WITH HARD-TO-KILL WEED.

Byline: JOSHUA SISKIN

Q: How do I eradicate oxalis from flower beds? I have tried all I can think of, and it keeps coming back. Any suggestions?

-- Esperanza Espinoza,

Reseda

A: Oxalis corniculata is one of the most attractive weeds in the garden. 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-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 flower beds, but shoots its seeds six feet in every direction. If you have a lawn nearby, oxalis will start growing in it. Oxalis in lawns is a bigger problem since its invasive growth in them can only be halted with chemicals.

How they get here

Oxalis and other weeds typically find their way into the garden by way of nursery plants. Carefully inspect nursery plants to make sure they are weed free before bringing them home. If you buy from a reputable nursery or garden center, you probably have nothing to worry about. However, many independent growers, such as some of those under power lines, are not always diligent when it comes to weed control, and landscapers may purchase the plants they put in your garden from these growers.

Many oxalis varieties

Oxalis is a plant genus that includes some truly ornamental types among its species. In fact, pink wood sorrel (Oxalis crassipes 'Rosea') is one of the most outstanding ground covers for our area. Pink wood sorrel grows best in partial sun, although it can take greater degrees of sun or shade, depending on the season. It grows from small bulbs (called bulbils) so that if it dies back from lack of water or inattention, it will soon make a fresh start when conditions for growth return. It blooms for months at a time and, although it spreads quickly enough through a garden bed, it is not invasive. Unfortunately, pink wood sorrel is not sold in local nurseries, although it is carried by mail order nurseries that are easily found by doing an Internet search. I found it at www.sunfarm.com and at www.bluestoneperennials.com.

Purple shamrock (Oxalis regnellii 'Atropurpurea') is noteworthy for its exotic foliage and ability to grow in considerable shade. Its oversize, deep violet, triangular cut leaves with pink striping are unique in the botanical world. Iron cross oxalis (Oxalis deppei) is a four-leaf shamrock with a maroon cross painted on its foliage. It has deep pink flowers and demands a sunny exposure.

Convolvulus 

Convolvulus is another plant genus that contains both a wiry-rooted weed and some highly desirable ornamental species. Field bindweed (Convolvulus arvensis) winds its way around the stems and leaves of everything in its path. It has attractive pinkish-white flowers but can never be completely dug out owing to its oxalis-like taproot.

On the plus side, bush morning glory (Convolvulus cneorum) and ground morning glory (Convolvulus sabatius) are bindweed relatives that deserve wider garden use. Bush morning glory grows into a 3-foot mound of shiny smooth silvery leaves covered with white trumpet flowers, while ground morning glory, growing into small mounds and trailing as well, has small, round green leaves with miniature blue trumpets.

TIP OF THE WEEK: If you live in Thousand Oaks, Chatsworth or Newhall and have heavy clay or other impermeable soil that needs amending, consider displacing it -- to a depth of at least six inches -- with topsoil (typically a fast-draining sand loam), which is inexpensively available by the cubic-foot bag at most nurseries and garden centers. Combine soil amendments such as compost or planter mix with the topsoil (four parts topsoil to one part amendment) and you should be able to grow most annuals, ground covers, and herbaceous perennials such as lily- of-the-Nile (Agapanthus) and daylilies.