This South African invader has charm. Its lemon-yellow flowers nodding over emerald leaves makes a lush foreground to the gray-blues of sea and sloughs. But any pleasure has long since changed to chagrin as carpets of buttercup oxalis began smothering the native plants and infesting our neighborhoods.
Sadly, the weed is so entrenched along California coasts that physically removing it from the wild just isn't feasible anymore. It doesn't spread by seeds but rather through small bulbils that develop on the stem beneath the leaves and by new bulbs that form underground along the rhizome. Each plant produces around twelve bulbils, so each year the buttercup crop significantly expands.
But take heart, home gardeners! With good timing, a bit of technique, and vigilance – especially vigilance – it's possible to reduce oxalis to manageable quantities within a few years.
One technique if you have limited numbers of the weed is to pull the plants gently out when they are big and lush but not yet in bloom. This is when the adult bulb is spent and the bulbils not yet fully developed. If you pull too early the bulb is still viable. Too late and all those bulbils are viable. Pulling is easiest in soft soil, so keep plenty of organic material on the ground to keep it conditioned.
A second technique, useful for larger infestations, is to exhaust the plants. Pull them up or sever their foliage repeatedly throughout the growing season until the bulbs are starved. Sever by mowing or by using a scuffle hoe or other scraping tool. Keep at it because until the plants run out of energy they regrow from shoots and bulb.
Important note: compost piles may not get hot enough to kill them, so bag up weeded plants and bulbs and put them in the garbage. Don't relocate or dispose of any soil where buttercup oxalis has grown as it is likely full of bulbs.
Tarping affected areas can be effective but is often impractical for home gardens as black plastic or shade cloth should be pinned over the affected area and left in place for at least six months.
As for herbicides, at least two recent research studies, one on a Mediterranean island and one in the Bodega Bay coastal area, have found glyphosate fairly effective. The former study used a 3% concentration and the latter only .13%. Glyphosate trials showed both significant oxalis reduction and rebound of native plants.
But whether or not glyphosate is a good option for home gardens depends on the type and severity of infestation as well as home owner preference. Herbicides are often not practical – and to many, undesirable – in the home garden, particularly as a first approach to removing invasives.
Unfortunately, there are no biocontrol agents available for buttercup oxalis at this time. Klugeana Philoxalis, a moth larva from South Africa that feeds on it, won't be available in the U.S. for years if at all.
In the meantime, battle the buttercups. They may seem overwhelming, but consider this: they are easier to remove, nicer to look at, and more pleasant to handle than the thistles and burs that otherwise plague us.