Recent sessions of not-really-much rainfall have greened our gardens and, inevitably, inspired weeds to grow.
If you are not already familiar with the “weed bank,” you must recognize that most garden soil has a hidden store of weed seeds waiting for the right conditions to germinate. Those weeds seeds do not demand a lot, just sun and moisture.
The rains provide the moisture, but the seeds must be close to the surface to gain access to sunlight. This condition can be met easily when weeds drop their seeds, winds transport them from faraway places, or birds drop them while fertilizing the Earth.
Some weed seeds are well below the surface, having been buried by soil tilling or erosion. They can survive long periods (the longevity varies with the species) until they are unearthed one way or another.
That’s one justification for “no-till” gardening, by the way.
Evidently, my garden had a shallow weed bank, because the rains brought an abundance of vigorous weeds in every area of the landscape.
When one experiences a seasonal burst of weed growth, the appropriate response is to weed the garden promptly, before the weeds set their seeds. One characteristic of weedy plants is that they reproduce enthusiastically. An old bit of garden wisdom warns, “One year of seeding leads to seven years of weeding.”
Long-term prevention of weed problems always begins with mulch. A layer of three or four inches of organic material serves shields sunlight from promoting the growth of weed seeds.
Another approach is the use of a pre-emergent herbicide based on corn gluten, which is a pelletized byproduct of the corn milling process. As a seed first germinates, it depends on nutrients stored in the seed, but as it grows it must develop roots to draw additional nutrients from the soil. Corn gluten is a natural, non-toxic material that suppresses a plant’s root development. It is most effective at the earliest stages of plant growth and has minimal effect on established plants.
Corn gluten treats all seeds the same, so it should not be applied when planting seeds of plants that you grow purposefully.
The downsides of corn gluten are that it is only about 50% effective when applied correctly. It requires repeat application whenever weeds begin to sprout.
Another downside is that when wet it will smell pretty awful for a while. One gardener friend who used this weed preventer suspected she had a dead body somewhere in the garden.
Also, corn gluten is rather expensive, close to $2.00/pound, perhaps because of low demand.
Finally, because most corn crops use Roundup for weed management, corn gluten almost certainly contains a residue of glyphosate, the active ingredient of this chemical herbicide.
After best efforts with mulching or pre-emergent treatment, and weeds are still growing, the traditional advice has been to pull them out by the roots. That seems gratifyingly thorough, but more recent advice is to cut weeds down, leave their roots to decay in the ground, and use their tops for mulch or compost.
That approach is sound, but only if done before the weeds produce seeds. There are also some weeds. Such as dandelions, that will regenerate from their roots.
One more thought: some plants that appear unexpectedly and in unwanted places in the garden, are garden-worthy plants that could be called “volunteers” or “self-seeders” rather than “weeds.” Examples include Italian lords-and-ladies (Arum italicum), various poppies (Papaver spp.) and the California Poppy (Eschscholzia californica), our state flower.
One attractive, not aggressive volunteer is the Hawkweed (Hieracium sp.), which I actually bought at the San Francisco Botanical Garden. This is a dandelion lookalike, with flowers very similar to the dandelion, but with unusual spotted leaves.
For more information:
Old Farmer’s Almanac: Common Garden Weeds
Fine Gardening: Six Tips for Effective Weed Control
Eartheasy: Corn Gluten Fertilizer (commercial product)