Given that our spring has been pretty mild over all, many gardeners are eager to get in the garden and plant a little earlier than normal. While the temptation is huge, it is also a good idea to remember that a hard frost can still come anytime in the next month or so and the ground is not all that warm. So what can go in the ground now? Lots of cold hardy vegetables and herbs are ready to go including onions, broccoli, cabbages, kales, mustards, boc choi, and more. It is best to harden off the plants for a few days before transplanting them - this means letting them get used to the cold, the wind, and the direct sunlight. Most of our plants will come to you already hardened off, but other nurseries may not take care to do that, so it is best to do it yourself. You can also use some row cover to create a small micro-climate for the young plants. We use hoops made out of 9-gauge wire to hold up the fabric off of the plants. This will give the plants a breathable, water permeable house to grow under during their initial weeks in the garden. The growth rate will be about double the rate of plants grown without row cover.
You can read more about hardening off here.
Another great thing to plant now are crops you direct seed and that can germinate in cold conditions - spinach, carrots, beets, turnips, radishes, scallions, peas, and fava beans.
If you have a spot in your garden that you are not going to use this year, you might want to consider spreading some cover crop seed. this will help ensure that the soil is fed during it's fallow period. The cover crop is essentially plants that are grown just for the soil's on going health. For cold weather sowing, you might want to try a cow pea and vetch mixture or oats or an annual clover . This will smother out weeds and add lots of organic matter to your garden.