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Stick to small or medium-sized flowering plants. Mix-and-match plants in large urns. Fill out your planter with ornamental grasses.
Gardening urns most often come in sizes that range between 12–29 inches (30–74 cm) wide and 12–24 inches (30–61 cm) tall. For this reason, they're best suited for planting showy annuals and perennials that don't put down deep roots. Many popular container species like azaleas, impatiens, and hostas also tend to do well in urns.  Dahlia, geraniums, impatiens, and other showy annuals are good for filling urns with bright bursts of color, while hearty perennials like blue star juniper or Creeping Jenny may be a better choice for gardeners with less time to devote to upkeep.  No matter what you choose to display in your urn, the most important thing is that the planter be about twice the size of the root system once it reaches maturity. Talk to someone at your local greenhouse or plant nursery or look up information online to find out how big you can expect a particular species to get. Oversized urns are typically spacious enough to house a variety of plants, making your companion-planting options nearly limitless. Try pairing 2-3 different offerings to put together your own unique arrangements. In addition to creating a more eye-catching display, combining multiple plants in the same container will help you save precious space in your garden and in smaller containers.  A cluster of peach-colored tulips, for example, will stand out in dazzling contrast to a bed of white heliotrope.  For best results, stick with plants that have similar soil, water, and sunlight needs, as they’ll all be sharing the same resources. If you have a little extra space to play around with after incorporating your main show plants, consider filling it out with some lush greenery. Grasses like fountain grass, fescue, and bluestem lend decorative urns a textural depth that can be used to balance out the bold colors of annual flowering plants.  Plant tall, spiky grasses in the center of the urn to serve as an impressive focal point, and sew cascading species around the perimeter of the planter for a more elegant overflowing effect. Most species of grasses are relatively low-maintenance, but they can compete for resources with your main display pieces, so make sure your arrangements get plenty of water and sunlight throughout the day.