what is the small, invasive, white-flowered weed blooming in April in...
Two white-flowered weeds blooming now are hairy bittercress and garlic mustard. Take a look at these in our weed gallery: http://extension.umd.edu/hgic/weeds/spring-weeds Control tips are included.
Hairy bittercress is smaller. Garlic mustard is a destructive non-native invasive plant--never let it go to seed. The seeds last for years and will continue to sprout.
For more information on Garlic mustard, go to Weeds Gone Wild website and look at the control options in the Plant Profiles section.
If neither of these plants is the one you are encountering, we'd suggest you send us photos.