Whacking annual weeds can kill them, if you whack them low enough—right into the dirt, below the crown. They have no means to grow without a crown, from which all their growth comes. (Chickweed is an exception, as it does not have a crown and will keep trying to grow and set seed until it has made enough of it.)
I saw weed whacking work several years ago, when the city or one of its maintenance contractors eliminated goat heads from the Wastewater Treatment Plant property for several years by repeatedly scalping the dirt where they grew with a string trimmer. This year, they are blooming and seeding in their old locations near the back fence; apparently the maintenance contract has changed and the city’s institutional memory is short, but goat head seeds can sprout for at least 5 years and will come up all summer until frost.
This year, I whacked the dense weeds along the irrigation ditch behind my mom’s house into the dirt in May, when they were still green and flowering, but hadn’t set seed yet. A week later, I pulled a few that had been missed. The next week, only the false dandelions had come back, and over two months later, that is still the case.
Last week, I was pulling and cutting crab grass from the backyard below, and looking at the enormous quantity of spotted spurge growing along the same path, but not yet flowering, and asked myself why I didn’t just whack them all, as the annual weeds would be killed out, while the perennial grass and clover would come back from roots. So I got my battery-powered trimmer out and started beating the dirt, working out which direction to work in order to throw the debris away from my legs and face. The next week, that area was still quite barren. This week, some were starting to grow back, and I whacked them again.
I will have to repeat all summer and reseed clover in the fall, I am sure. Spring weeds start in the fall and winter, flowering and setting seed around the same time in the spring, allowing one to kill them out for the season with one soil-scalping before they are ripe. Some summer annuals, like star thistle, start together in spring and can be killed out as they start to bloom in early summer. Most summer annuals keep sprouting through the hot weather, particularly where they are watered. Goat heads have a final flush of sprouting as the weather cools in the fall, just as heron’s bill is starting its fall sprouting season.
What does not work to kill weeds, of any sort, is cutting two inches above the ground. They continue to spread their flowers and ripen seed below the cutting height. Weed control is seed control, and if the plants are still flowering, they are still violating our city code, which requires that one prevents flowering, seeding and noxious growth by cutting or killing.
Rycke Brown, Natural Gardener 541-955-9040 firstname.lastname@example.org