Annual
weeds persist as resilient seeds in the soil. Field speedwell
seeds, for example, may last 30 years or more, although others
like groundsel last for just three years. They germinate when
they experience stimuli such as warmth, fluctuating temperatures
and especially light, that indicate good growing conditions.
Weeds have different requirements for germination that define the time of year that they grow. For instance annual meadow grass requires warmth, light, moist soil and soil nitrates after a period of cold weather, making it a spring weed.
To
be successful annual weeds have to shed numerous seeds and
therefore have very small seeds. A large groundsel plant may
shed 38,000 seeds! Annual weeds are thwarted by mulches at
least 5cm (2in), but ideally 7.5cm (3in), deep of organic
matter, such as cocoa shells, bark or mushroom compost or
by mulching sheets of dark material. Mulches exclude light
and any germinating seeds have insufficient reserves to grow
through the mulch or seek out any planting holes in the mulch.
Another strategy where mulches are inappropriate, on vegetable plots or areas of bedding plants for example, is to allow weeds to germinate before planting or sowing, and then eliminate them with very shallow hoeing or with a weedkiller. This creates a 'stale seedbed' where relatively few weeds will subsequently germinate.
Any
weeds that do survive should be hoed or pulled by hand. Annual
weeds must never set seed. Any seed set will replenish the
seed bank of weed seeds in the soil ensuring any weed problems
will persist. Don't forget the old saying: 'One year's seeding
gives seven years weeding'.