2. Assemble the Plants
Think beyond one-note plantings of marigolds or impatiens. For a long-­lasting display, combine showy annuals with ornamental grasses, shrubs that change leaf color in the fall, or dwarf evergreens. Go for a mix of colors, textures, and foliage types. A tall grass, a delicate vine, and a plant with large, interesting leaves make a good combination. "A vertical element is important to give the planting height," says Gabrielle Whiton, a container-plant specialist at Bainbridge Gardens, a nursery on Washington's Bainbridge Island. She often starts with a dwarf conifer, then selects lower-growing and trailing plants to go around its base. One of those might be a flowering perennial or annual in a 4-inch container. She places this, pot and all, into the soil at the front of a large container. Once the blossoms fade, she can pull out the small pot and put in a new one with a plant just coming into bloom. She also likes to echo the pot color in her plantings.

To balance form and proportion in a pot, Ellen Zachos, owner of Acme Plant Stuff in New York City, which creates and maintains container gardens, relies on her own catchy recipe of "thrillers, fillers, and spillers." Thrillers are tall plants that go in the center or back, fillers are medium-size plants that fill out the middle, and spillers gracefully trail or cascade over the edge to soften the pot's hard edges."Resist the urge to crowd in too many different things," says Ellen ­Zachos, who likes to stick to three to five types, tops. "A lot of plants are fine, but a lot of different kinds of plants starts to look messy."

Unless you need a deck decoration for a party next weekend, select plants that are relatively small and let them fill out. Avoid buying ones whose roots stuff the nursery container. Dense root balls shed water, so these plants may become parched once you repot them, even if you water often. Pair up plants that are suited to the same exposure, whether sun, part sun, or shade, and that have similar water requirements.

3. Deal With the Dirt
Since ordinary garden soil is too heavy and can introduce disease, be sure to use a bagged planting mix or a homemade equivalent. Products labeled "potting soil" contain sterilized soil and other ingredients, while "soilless ­mixes" consist mostly of peat moss or peat substitutes, compost, and perlite or vermiculite to keep it loose. Soilless mixes weigh less but dry out faster, but some plants, such as succulents, prefer them. For a homemade batch, mix five parts compost with one part builder's sand, one part vermiculite or perlite, and one-quarter part dry organic fertilizer. Whichever medium you use, check to see if it contains slow-release fertilizer; if not, consider mixing in some granules at planting time.

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