i planted fall beets and lettuce..now it’s going to freeze tonight…what do i do?
You did not mention the stage of the beets and lettuce. A hard freeze can turn the foliage to mush. If your beets have formed storage roots and the lettuce has formed a root system, you should harvest them. Most leaf lettuces can be harvested when 5-6 inches tall. Otherwise, provide some frost/winter protection and you will have to take your chances. You can cover the plants with a floating row cover which is a spun bonded fabric that provides some frost and insect protection. Row cover comes in many widths, sizes, and weights. Heavy-weight FRCs are usually used to extend the growing season in spring and fall, allow 50%-70% light transmittance, and 4º-10ºF. of frost protection. http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/not_updated/GE004Flo...
If you do not have a row cover you can cover with a heavy plastic but remove during the day for good air circulation. Strong sunlight can cook the plants.
See our Vegetable Profiles on beets http://extension.umd.edu/learn/vegetable-profiles-beets and lettuce http://extension.umd.edu/learn/vegetable-profiles-lettuce
September-October is really too late to plant beets in Maryland. Leaf lettuce should be in the ground by the beginning of September.
See our planting calendar in our Publications > Vegetables: http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/not_updated/GE007_Ve...
Winter crops do not so much grow during the winter, as they can survive with protection if they are well established already.
Here is a good chart, specifically for Maryland, to help schedule planting: http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG16%20Planting%20Da...
Winter crops (beets, spinach, tough lettuce such as endive, escarole, arugula) generally need to be in a tunnel of heavy row cover, or a cold frame, or very heavily mulched.