Weed Strategies

Yesterday was the first Friends of the Nature Park Cleanup since the spring and despite the hot weather, there was a nice turn out. During the summer some people visited the park to remove weeds and litter, but for the most part it has been on its own.

Native plants thriving in the Nature Park. Buckwheat (Eriogonum fasciculatum) flowers in front.

And I am happy to report that it is looking pretty good. Yes, it is weedy. Yes, many of the plants we put in last winter have died. But seeing young deerweed (Lotus scoparius), white sage (Salvia apiana), black sage (Salvia mellifera), buckwheat (Eriogonum fasciculatum) and California fuchsia (Epilobium canum) growing in what are clearly very difficult conditions is encouraging. Some of these will make it to maturity and will propagate by seed. Over time it is our hope and expectation that as the soil develops, disturbance becomes less common, and native plants establish, the weeds will decline.

The dominance of weeds in highly disturbed sites is a major problem in most urban settings. Our strategy for weed control is based in part on the Bradley method of bush regeneration (Bringing Back the Bush by Joan Bradley, 2002) developed by Joan and Eileen Bradley in Australia. Their strategy is to work meticulously from good habitat out, disturbing the soil as little as possible. In the So Pas Nature Park there is little good habitat to start from, but we are trying to keep the weeds away from clusters of native plants. New plantings of shrubs and herbaceous perennials were mostly located next to areas with successful native plant growth, rather than spread throughout the park.

The second strategy that we follow is to know our enemy and attack its weakness. We concentrate on removing immature seedlings before they form flowers. For example, castor bean (Ricinus communis) is a perennial that seeds heavily. We focused on removing mature plants, cutting and bagging seed heads, and extirpating seedlings during the wet season, removing all immature plants before they can flower and reseed. Though this was one of the most pervasive weeds in the park when we started, there are very few left.

Castor bean (Ricinus communis) easy to remove as seedlings.

A second pernicious weed that we have controlled is tree of heaven (Ailanthus altissima). There was a large tree at the east end of the park, south of the big western sycamore (Platanus racemosa). When the park was relandscaped, the tree was removed. Unfortunately the tree’s extensive root system was left in place and it continues to sprout new trees. Any sprouts allowed to remain photosynthesize, producing food for the remaining roots. We now remove all sprouts so that over time the roots will shrink and eventually die.

The weed that I went after on Saturday is horseweed, Conzya species. It is just now flowering and going to seed. We cut and bagged the flower heads. The rest of these annuals (sometimes biennials) were dislodged and allowed to decompose. It was really hard to bag all of the seed heads and I think a better approach may be to leave those that are already flowering and just remove immature plants.

Flowering horseweed (Conyza species).

Flowering horseweed (Conyza species).

Flower heads cut and bagged.

Plants with no flowers/seeds are uprooted and allowed to decompose in place.

Expanse with horseweed removed. The beige weed is mustard that has already gone to seed. Next year we will try to remove the seedlings before they flower.

Russian thistle, or tumbleweed, (Salsola tragus) is another annual that spreads by reseeding itself. If forms a large rounded plant that breaks from the root system once the seeds are mature. The entire plant then rolls along as the wind blows it, spreading seed great distances as it goes. There were some plants in the park. Unfortunately they had already formed seed. We will keep a look out for these next year right as they begin to grow, removing young seedlings before they flower.

Tumbleweed that is still green but has already formed seeds. There is too much of it to bag and remove. From now on we will try to remove immature plants.

I hate to end a post with an ugly weed picture, so here is young white sage (Salvia apiana) that is defying the odds by making it through its first hot, dry summer with no supplemental water in weedy, degraded, compacted soil.

Go Natives!!