January and February timely terms (36518 bytes)

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Timely Terms for Garden Planning

Hardiness Zones

Annual, Biennial, Perennial–What’s the difference?




Hardiness Zones are areas in the United States that share the same average winter temperature: the lower the number the colder the area. Knowing a plant’s zone rating helps gardeners determine if it is hardy enough to survive the winter. The Denver-area is Zone 5. That means a plant labeled as hardy to Zone 3, 4, or 5 should survive here. Remember, hardiness zones are rules-of-thumb, not guarantees!

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Annual, Biennial, Perennial–What’s the difference?

  • Annuals are plants that complete their life cycle in one growing season: from seed, to plant, to flowering, to seed production. The plant dies at the end of the season. Many annuals are self-sowing–that is, the seeds they produce start new plants near the same place the following year.

  • Biennials are plants which grow through one season, bloom the next season, then die. .Like annuals, many self-sow.

  • Perennials are plants which grow during the spring and summer, remain dormant during the winter, then grow again the next season. Some, like peonies, are very long-lived. Others, like penstemon, may last only a few years. Some may not begin to bloom until the second or third season after planting.


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Microclimates are areas within a landscape that differ from the rest in temperature, sun exposure, and moisture. Since plants respond to these three factors, location makes a difference. Microclimate may explain, for example, why a Zone 6 plant which "shouldn't" do well in Denver, thrives in a certain part of your garden, or why a plant your neighbor grows does not do well in your yard. Identifying the microclimates on your site will help you put the right plant in the right place.



Southern exposure Most sunlight. Earliest growth and flowers. Possible winter-damage to broad-leaf evergreens. Hot, dry conditions near walls.
Eastern exposure

(morning sun)

Generally good gardening sites. Especially good for roses and broad-leaf evergreens. Somewhat cooler than south or west. Part-shade in afternoon.
Western exposure

(afternoon sun)

May be warmer than other exposures due to sun intensity and radiated heat. Drying winds may be a problem. Good for plants that thrive on intense sunlight and dryness. Other plants may need more water and mulch.
Northern exposure Little direct sunlight. Plants start later and end season sooner. Not as much temperature fluctuation as southern and western. Shade-tolerant plants do well.
Slopes North-facing provide protection to plants below. South- or west-facing may be very hot and dry.
Water, including pools Moderates temperatures.
Reflected heat from buildings, streets, driveways Creates hot, dry conditions.


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Date last revised: 01/05/2010