Woodsorrel has greenish or reddish colored leaves and yellow flowers. Bittercress, a member of the mustard family, has white flowers and cigar-shaped fruit. When the fruit of woodsorrel and bittercress are mature, the seed can be thrown several feet as the fruit ruptures. This is how these weed species can rapidly infest a greenhouse. Weed seed can also enter greenhouses through infested media or nursery stock.
Sanitation is the best way to manage weeds in greenhouses.
Hand weed pots before bringing plants into a greenhouse. Control weeds growing outside the greenhouse to prevent seed from blowing into the house. Remove weeds from the house before they have a chance to flower. Media can be steam sterilized to help with weed control, although some weed seed will not be killed. Maintain the media at 180 F for at least 30 minutes when steam sterilizing potting mix.