Improving the Balance in the Soil Seed Bank

“Even the weeds look lovely in May.” From Arthur Lee Jacobson in his book Wild Plants of Greater Seattle.

April comes and goes. The rainy season begins to wind down, but the soil remains moist. Days grow longer and highs begin to flirt with 60 degrees. Nighttime lows no longer drop into the 30’s. All three factors combine to help trigger the germination of seeds. By the middle of May, a flush of new growth carpets the ground, especially around the edges of the forest if the soil has been disturbed at some point.

Soil Disturbance Triggers Seed Germination

Each time we pull a weed out of the ground the soil is disturbed, and one or more new plants usually sprout in that spot. Why? One possibility is that the soil disturbance triggers germination by enabling light to reach seeds that were previously buried in darkness. The types of new plants that sprout depend partly on what seeds are lying dormant in the soil. If the “soil seed bank” contains mostly weeds, then weeds will be the likely replacements. The more native species represented in the seed bank, the greater the odds that they will take the place of the weed that was removed. Over time, through careful hand weeding, patches of weeds are succeeded by patches of native plants. Bad edges become good edges. Fewer undesirable plants spread into the adjoining forests.

Rampant Weeds

The process described above can take several years of persistent hand weeding. The seeds from Herb Robert (a Class B Noxious Weed in Washington) remain viable in the soil for five years or more. In a sunny area, a single Wall Lettuce (Mycelis muralis) may produce more than 10,000 seeds. Nipplewort (Lapsan communis) is an attractive green, until it shoots up its leggy flowering stem and produces masses of fluffy seeds that float away at the slightest disturbance.

Rampant Native Plants

There are native plants as well, that can spread rampantly if given half a chance. For example, Cicely (Osmorhiza genus), a perennial that spreads readily by seed and can take over patches of disturbed ground. In some years the annual Candyflower (Claytonia sibirica) forms dense patches that deter other plants. I have made my peace with Cleavers (Gallium genus) and allow it to grow in many areas. Some native plants like these get leggy and unattractive by the end of the growing season, but from an ecological perspective they are more desirable that non-native weeds. Over time, in “showcase” garden spots, I try to replace them with more attractive native plants, but in the interim, I am delighted to see them out-competing weeds.

The need to weed never ends, but as desirable plants take over, the job becomes easier. With fewer weeds to remove, the ground is less disturbed, and fewer germinations are triggered in the first place. And, with the changing balance in the soil seed bank those germinations are more likely to be natives.