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Selecting, Buying, and Planting Annuals and Perennials

By Laura Pickett, Colorado State University Cooprative Extension , greenhouse specialist

The arrival of bedding plants marks the beginning of spring, even for the most casual gardener. Here's a to-do checklist before purchasing plants.

Know the difference between annuals and perennials. Perennial plants grow and spread for years. The initial cost is greater, but they are a permanent addition to the garden. Plant tall perennials at the back of a border and shorter plants in front. With a little planning, it is possible to plant masses of successively blooming flowers for an all-season display. After three years, most perennials need dividing, giving you and your friends hundreds of new plants.

Annuals are a one-season splash of color. They bloom almost immediately and continue blooming until the first killing frost, which often occurs anywhere between October 15 - 20 in the Denver metro area.

Annuals are maintenance intensive. To continue blooming, they require close attention to watering, fertilizing, weeding and deheading. Annuals often are used in new landscapes as colorful fillers, while foundation shrubbery grows and matures. They also can be planted in hanging baskets and containers adding color to a deck, garden, or patio. Annual plants often are more costly in the long run than perennials, because they must be replaced each year.

The average frost-free date for the Denver metro area is between May 5 and May 15. If tender annuals are planted too early, a late frost could kill every plant. Perennials and hardy annuals such as pansies and snapdragons will survive a light frost.

Prepare the soil well before you plant. Colorado soils can be heavy clay or very sandy. The structure of either soil type can be changed by adding organic matter such as Canadian sphagnum peat moss, well-rotted manure or garden compost. Spread two to three inches of the organic matter over the soil and rototill or double-spade it in. Add a well-balanced fertilizer at this time.

Don't take short cuts with soil preparation. When planting a perennial garden, you have just one chance to do it correctly.

Select plants that are suitable to the selected site. Most garden centers will list the specific exposure, soil and cultural requirements for a variety of plants. In the absence of signs with this information, read the tag on each variety. Note the amount of sun the plants will receive. Is wind a factor? Many tall flowers do not tolerate wind and snap easily; other tall varieties bend with the wind and perform beautifully. Wind can damage shorter plants too. Impatiens, for example will shrivel and die in a windy location because they dehydrate easily.

It's time to buy the plants. Avoid cell-packs containing tall spindly plants with flowers. They've been in the packs too long. You can pinch them back and trim the roots for better performance, but this sets them back, and you'll wait longer for blooms than if you'd chosen smaller plants. Check leaves and stems for diseases such as leaf spots or stem cankers (dark, sunken areas) and insects. Take the plants out of the pot or pack and check the roots. Healthy roots should be firm and white, without spirals or kinks. Don't bring home unhealthy or insect-infested plants.

If plants are purchased directly from a greenhouse, they'll need to be hardened-off. Move them outside for longer periods of time each day until they can remain safely outdoors.

Water and drain cell-packs for 5 to 10 minutes before removing the plants. Remove transplants from cell-packs by gently squeezing the bottom of each cell. Take out the plants one by one to delay drying of the root ball. Pinch off long coiled bottom roots with a garden knife and gently rough up root-bound areas. Arrange the plants close enough together to make a full, attractive bed.

As a general rule, don't plant seedlings deeper than they were in the cell-pack. Firm the soil lightly around the plants and water thoroughly with a soft spray nozzle attached to a hose. Transplant shock can be minimized by planting during cooler times of the day, preferably evening. Protect newly planted seedlings from wind for a day or two.

Photograph courtesy of Judy Sedbrook.

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Date last revised: 01/05/2010