If you had walked through GFE in March five years ago and looked uphill into the orchard, you would have seen a solid sea of yellow flowers foaming around the trunks of our fruit trees.

These were the flowers of Oxalis pes-caprae, or as San Francisco kids call it, sour grass. This oxalis is an invasive exotic, originally from South Africa. Thriving in acidic soils and a summer-dry climate similar to the one of its origin, oxalis has spread vigorously throughout Northern California to the disgust of gardeners and the caretakers of our wild and native plant communities. We do have a native oxalis, which belongs in the understory community in redwood forests, but that one does not have a yellow flower.

A combination of factors allowed our oxalis to become a weedy explosion. The steep, eroding, sandy hillside was difficult to access and work on, so we rarely weeded up there. The orchard trees began receiving fertilizer and summer water, which strengthened the hold of the oxalis spreading below them. And our soil is slightly acidic, which favored the oxalis but not the orchard trees. Most of our fruit trees access soil nutrients best in a sweeter, more basic soil.

The last straw was when someone from the Native Plant Society dropped by to complain about our oxalis plantation. They were concerned because GFE is adjacent to some of the last remaining wild Franciscan plant communities, up behind Laguna Honda reservoir. Oxalis would be a very unwelcome exotic competitor in that area.

So something had to be done. What’s a restorative garden team to do?

Of course, as an organic garden where no pesticides have been used for over twenty five years, we didn’t consider spraying roundup on the oxalis, but let me just remind everyone that the oxalis would have returned when conditions are so good for it, and then we would have been caught in a cycle of spraying and resurgence. Great for roundup sales, but not so great when we’re gardening for the environment.

So we made a long term plan to change the conditions in the orchard so that they no longer favored the oxalis. More painstaking and slow, but a permanent fix that we could benefit from in lots of ways.

Our first step was to stabilize the eroding hillside, and provide access for more frequent maintenance. For the next several years, our time in the orchard was spent building a curved retaining wall in front of each fruit tree, which provided a stable level area for roots to thrive, and for us to be able to apply nutrients and water to each tree more effectively. A second benefit to the fruit trees was that we built these walls out of recycled broken concrete from urban sidewalk renewal. This ‘urbanite’ is a free resource for us, and uses concrete that would otherwise be headed for landfills. Installing all that concrete into the hillside is slowly raising the pH of our orchard soil, which benefits the fruit trees and discourages the oxalis.

Newly constructed “urbanite” rock wall tree basins in the north orchard at GFE.

Now that the hillside was more accessible and stable, we started a “take no prisoners” weeding program, trying to create a weed-free, clean orchard floor. After weeding, we mulched with compost sticks, to add organic matter and build the microbiome in the sandy soil. But the oxalis was still growing faster than we could weed it out or smother it. And the constant disturbance of the soil was promoting erosion rather than reducing it.

So the final step in our orchard makeover has been to begin planting the orchard floor with plants that we do want, for their beauty, or compatibility, or for a second harvest from the same area. We are concentrating on perennials which will grow a little taller than the oxalis and shade it out, or form a tough mat or roots and stems to out-compete it. So far we are planting native yarrow (for beautiful flowers that attract pollinators and other beneficial insects), sand strawberry (which forms a net over the soil, further preventing erosion), and catmint (beautiful flowers, a great oxalis killer, and a second harvest for kids to make cat toys with). This year I would like to add daffodils, which discourage gopher activity. Gophers do not respect our project, and they have eaten most of the catmint!

New plants in the north orchard marked with purple flags.

But our staff and volunteers do respect our project, and the purple flags are multiplying in the orchard to mark where we have planted little starts and divisions of our new orchard floor plants. This way, we won’t trample them while weeding.

The story of the purple flags continues next month. How do the purple flags fight climate change? And “what’s wrong with oxalis?” asks Dr. Christine Jones.

Garden Tips

Why the Purple Flags?