In California, Oxalis pes-caprae has spread along the coastal length of the state, with some occurrences in the central valley and foothills[i].
It has grass-green clover-like foliage and bright yellow flowers. Sprouting from a basal rosette, the leaves appear at the end of leafless stalks that can grow to about 10” in length. The flower stalk usually rises slightly higher than the leaf stalks and sports multiple almost fluorescent yellow flowers. We’ve seen the stem, which begins at the rosette and extends below ground grow as much as 12” underground or as little as half an inch. It ends in at least one bulb, or sometimes will have a string of bulbs attached to the “mother” bulb, each deeper than the next.
This pesky perennial sprouts with the first winter rains and tends to die back with the warmer and drier months. It doesn’t create viable seed in California but one plant produces about 12 bulbs in a single growing season which each can grow into a new plant[ii].
Oxalis readily increases in undisturbed sand dunes, but since it only spreads by bulb it generally colonizes new territory through the movement of infested soil, so be sure not to transport soil that may have oxalis bulbs in it[iii]. Gophers and moles also move the bulbs into new areas.
Manual removal is most effective when the entire plant, including the bulb at the base of the stem, is removed. Unfortunately, this can be very difficult because the stem, which often extends fairly deep underground, easily breaks from the bulb when pulling the plant from the ground. I’ve found digging down with a pick, butter knife or spade to try to get underneath the bulb itself can be helpful. This also loosens the dirt and can make gentle removal of the entire plant easier.
If the bulb stays in the ground, the plant will come back, but according to the Weed Research and Information Center at the University of California, pulling the tops of the plant regularly will eventually weaken the bulb by draining its carbohydrate reserves[iv]. Even though this method can take years to be effective, it’s better than nothing.
If you have an infested area, try manually removing as much of the plant as you can, including the bulbs. Then place cardboard over the area, making sure to cover the entire area and not leave any gaps. Then spread 2-4” of mulch to try to suffocate the weed. Wait at least one season, then you can either cut through the mulch and cardboard to plant your desired plant, or layer fresh soil over the mulch and plant on top of it.
This strategy is obviously best for plants with a shallow root system. Our annual wildflowersperform well with this method, until the cardboard has decomposed. It can be helpful to plant a vigorous perennial or annual over the area to outcompete the Oxalis as it makes a comeback. If the oxalis is nestling between your favorite plants and you don’t have the luxury of taking an area of your garden out of commission for a season or two, then you’ll have to manually pull the weed and stay on top of it.
With perseverance, you will get ahead!
[i] The Cal Flora Database. Accessed 1/19/15.
[ii] UC Integrated Pest Management Program. Accessed 1/7/15.
[iii] California Invasive Plant Council. Accessed 1/19/15.
[iv] Weed Research and Information Center, University of California. Accessed 1/17/15.