Here's the announcement from their site:
4th Annual DC Plant Swap: H Street NE
Plant swap hosted by Washington Gardener Magazine; bring and receive free plants to expand your garden! Please bring your plants for sorting by category (shade perennial, groundcover, herb, etc.). Do not bring any common orange daylilies and any invasive species. Also bring a name tag and a pen and paper (you will want to take lots of notes as folks describe their plants and growing conditions). Pot plants up NOW -- the longer they can get settled in their pots, the better their chance of success and survival. Please fully label all your swap plants with as much info as you have, optimally that will include: common and scientific name, amount of sun needed, amount of water needed, any other special care notes, and color of the blooms (if it is not currently in flower). What if you do not have plants to swap? Come anyway! Bring refreshments like cold drinks and yummy finger foods to share with the other swappers. Be sure to also bring cups, napkins, utensils, serving spoons, etc, if your food item requires those for serving.