Annual Winter Weeds: "Dr. Z's" Tip of the Week

Winter annual weeds show up very early in the spring, during a "normal" season.  This year they are just starting to show up now due to the ongoing cold weather.
These weeds cause damage early in the season, when it's cool and moist.  They disappear when it's hot and dry, only to start up again in the late fall when it's cool and moist.  Because of this they are among the most difficult weeds to prevent or kill.  The problem is one of timing, since it's very weather dependent, and also the short window of opportunity to prevent them.  Prevention is none the less the key, because once the weeds are killed the damage has already been done.
Early winter annual weeds are extremely destructive and they cause their damage quickly.  They also spread very fast and become hard to control.  Part of the problem is that this happens so early in the spring, before we start to work in the garden.
The general cycle of these weeds (starting at this time of the year) is as follows:  There is an actively growing weed now that it is cool and moist which is full of seed that it will spread.  As soon as it warms up the weed stops growing and begins to fade.  When it's hot and dry the weed dies completely leaving all the seed behind.  The seed lies dormant all summer and early fall.  As soon as it cools off in late fall/early winter, and moisture is introduced, the seed germinates and the small weed winters over.  Sometimes the conditions are not right at that time, and the weed will not germinate until late winter/early spring.  Soon there's a mature weed causing damage and spreading even more seed.
Two of the more "nasty" annual winter weeds are annual bluegrass and bittercress.
Annual Bluegrass: This grassy weed is light green in color with prolific seed heads.  It forms dense patches in both lawns and garden beds.  In a lawn it starts out looking healthy, but much lighter green than the rest of the lawn around it.  As the weather warms up, it slowly turns yellow until the hot weather arrives,and it turns brown and dies leaving big dead areas in the lawn.
Bittercress: This broad leaf weed is a winter annual in this part of the country.  Once it germinates it forms a small basal rosette of leaves.  It has very few upper leaves, and sends up a flower stem with white flowers at the top.  These produce seeds which ripen quickly and can be propelled up to 12' away from the plant.  The spread of this weed can be slowed down by cutting, and then collecting, the flower stems before they go to seed.