San Francisco Chronicle - (Sunday)
How to kill weeds so your plants will grow
Keeping plants alive: That’s what gardeners do. Right? But the wise gardener also knows how to kill plants — specifically weeds. A slapdash, distracted approach to killing weeds is likely to result in their multiplying rather than dying. Here are tips that will help you free your garden of plants you don’t want so the ones you do want can thrive.
Weeds are at their worst in a new garden, one that is just getting started, and in a vegetable garden, where seeds and seedlings are often being planted in freshly prepared soil.
They are especially damaging to vegetable crops because most crop plants are fastgrowing, greedy in their need for light, water and nutrients.
Most food gardeners prefer to avoid herbicides for environmental reasons, not to mention being concerned with human health hazards. Also, herbicides often get rid of only the tops of weeds — a cosmetic improvement — while allowing seeds and underground reproductive structures to live, so the weeds return rapidly.
A smart gardener starts by learning the names of the weeds that have invaded, then learning when and how they reproduce (see the weed chapter of my book “Golden Gate Gardening”). If the weed is an annual, a plant that regrows only from seed, preventing seed fall is critical. If it is a perennial that regrows from underground structures, you can kill it by removing those structures. (Some perennials drop seed as well.)
Removing weeds by hand when they are very young is always a good idea. You will find this especially important in a seedbed, where the weed seeds will germinate freely in the prepared and watered bare soil. Remember, when you do this, that the weed closest to the seedlings you are trying to grow competes more grievously with them than the ones growing farther away.
Quite a few annual weeds are accidental im
Weed control
When pulling weeds, it is generally best to do it a plant at a time. If you try to grasp a handful, you will do an imperfect job of pulling some of them and may have to go over the soil again to remove stragglers.
In areas that will not be disturbed for a season or longer, a mulch will help control weeds. For vegetable beds, a fine-textured mulch is best, one that will slowly decay and amend the soil. On paths or between shrubs, use two layers of brown box cardboard covered with an inch or two of woodchips. (Avoid nonorganic weedblock cloth, as it is petroleum-based, and also because some weeds grow roots through it.) Weeds in cracks of rock or pavement can often be removed with special tools (ask at your garden center), or may be killed by pouring vinegar or boiling water into the crack. ports from Mediterranean Europe, so they start to grow in the fall, are watered by the rains, then flower and set seed in the spring. Gardeners often rush out to pull them when weather warms in April, but by then, seeds have already dropped. And, because weed seeds tend to germinate gradually, over several years, letting them fall will ensure new weed plants will appear for years to come. Better to brave the relative chill to remove immature coolseason weeds on a fall or winter day. A few annual weeds, such as purslane and lambsquarters, start to grow in late spring in watered gardens, so should be pulled before mid to late summer when they’ll drop seed.
Pulling an annual weed, such as chickweed or groundsel, is enough to kill the plant, but if the plant is a perennial, you generally need to dig it out to kill it. A dandelion will regrow unless you dig out most or all of its taproot. Devil bulbs (Nothoscordum) will regrow from tiny bulbs in the soil.
In some cases, repeated removal of the visible part of a perennial weed will eventually kill it. One example is bindweed, a summer pest. Pulling off leafy tops repeatedly before they are 3 inches tall will eventually kill the plant. Another example is Oxalis pescaprae. Removing tops repeatedly in fall will prevent it from forming more bulblets each year and should, over time, lead to death. These are good options in planted beds, when you don’t want to disturb growing crops or when there are so many underground structures that removing them one by one seems futile. (No one has ever proved that yellow oxalis forms seeds, but bindweed and many other perennials do, so the bottom line is to remove all weeds before they flower.)
Pam Peirce is the author of the regional gardening book “Golden Gate Gardening,” for mid and Northern California locations both coastal and inland. She blogs at goldengategarden.typepad.com, where she lists appearances and often writes more on topics covered in this column.