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Usually controlled by timely cultivation and mulching
Pigweed (left) and lambsquarters (right) are summer annuals
Example: the Pigweeds Smooth pigweed (left), spiny amaranth (center), and Palmer amaranth (right) emerge in late spring, triggered by high, fluctuating soil temperatures. They respond to high soil N, set viable seed ~10 days after pollination, and die with frost.
Broadleaf versus Grass Broadleaf seedling growing point above ground – easy to kill by shallow cultivation or flaming when weeds not more than 1 inch tall. Grass seedling growing point below ground – harder to kill by cultivation; flaming ineffective; cultivate as soon as they emerge.
Step 2: Minimize Niches for Weed Growth Open niche in time: soil left exposed between harvest of one crop and emergence of the next crop is an open invitation to the weeds.
Open niche in space: bare soil between crop rows requires cultivation for weed control until the crop canopy closes.
Cover crops fill post-harvest niche Timely planting of winter rye + hairy vetch after vegetable harvest (left) fully occupied the bed, while common chickweed did the job when no cover was sown (right).
Relay planting closes post-harvest niche more rapidly Clover cover crops were interseeded into brassicas (left) and tomato (right) when vegetables were at mid-growth. After vegetable harvest, clover rapidly covers the ground.
Mulch partially closes weed niches An organic mulch effectively closes the bare soil niche for weed seedlings. Established grasses or perennial weeds will break through.
Carrots are weed-prone, but the ‘Danvers’ type varieties have more vigorous tops that close canopy within 60 days, aiding weed control.
Season Extension Summer squash (left) and snap bean (right) were grown for an early market under low tunnels (covers now removed). Good season extension technique maintained near-optimal conditions for the crop, and thereby facilitated weed control.
Feed and Water the Crop, Not the Weeds In-row drip irrigation waters lettuce (left) and tomato (right) while leaving between-row weeds dry.