Fast-spreading perennials will quickly fill in an area, sometimes even choking out surrounding plants. For the impatient gardener who wishes to start perennials from seed, these are some of the best -- because as long as the conditions are right, they will germinate and begin to grow in 30 days or less. Most perennials take two to three years before they will flower when started from seed. These perennials will flower between 12 to 20 weeks from the time the seeds begin to sprout.
Rapid Ground Cover
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There are a number of fast-spreading perennial groundcovers that, if not kept in check, will literally take over an area. Common periwinkle (Vinca minor, USDA zones 4-9), bugleweed (Ajuga reptans, zones 3-10) and lamb's ears (Stachys byzantina, zones 4-9) are practically unstoppable once established.
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Digging and removing the plants is the best way to get rid of the ones you don't want. These plants do a good job of choking out weeds. Plant these ground covers in an area of your garden with established plants, such as trees or shrubs, because they may spread too aggressively in your fast-spreading flower garden.
Low-Growing Perennial Edging
Using fast-spreading perennials is a great way to establish a floral low-growing perennial border. Choose low-growing plants so they don't hide the plants growing behind them.
Sea thrift (Armeria maritima, zones 4-8) reaches a mature height of 6 to 12 inches. Alpine aster (Aster alpinus, zones 5-7) also reaches a mature height of 6 to 12 inches. Cheddar pinks (Dianthus spp., zones 4-9) reach a mature height of 12 inches. Many of these plants will self-sow, in addition to spreading by underground roots.
Perennials for Center of Garden
The center of the perennial flowerbed is fairly easy to access throughout most of the gardening season. Plant fast-spreading perennials that will bloom all season long in this area, since you will easily be able to reach them to remove the faded flowers. This process is known as deadheading. Space these plants far enough apart so they do not overtake one another or the front border.
Good choices for the center of a garden area are rose campion (Silene coronaria, zones 4-10), black-eyed Susan (Rudbeckia spp., zones 3-9) and purple coneflower (Echinacea purpurea, zones 3-8).
Perennials for Back-of-the-Border
Fast-spreading perennials planted near the back of the garden border can be difficult to access. Some of these perennials will bloom again if you deadhead them, but if you cannot easily get to them, don't worry.
Tall garden phlox (Phlox paniculata, zones 4-8), delphinium (Delphinium spp., zones 5-9) and bee balm (Monarda spp., zones 4-9) are all fast spreaders, especially if the soil conditions are right.
A good way to encourage perennials to spread fast is to top dress the soil with 3 inches of compost in early spring. Another way to encourage them to grow faster is to provide regular irrigation. An inch of water per week is ideal.