Putting Calgary’s Gardens to Bed

Summary notes prepared by the Calgary Horticultural Society:


Glynn Wright, Bert Einsiedel, Alice Anderson and Janet Melrose


1: Calgary’s Climate and microclimates

  • Calgary has six months of winter, and contains plant hardiness zones 2 to 3b, with occasional pockets of warm zone The expression “frost free days” has to be refined to indicate consecutive frost free days during our growing season. The last frost typically occurs on May 23 and the first fall frost on September 15, giving an average of 115 consecutive frost free days; however, this varies from about 50 days to about 150 days. 5°Celsius is often the minimum temperature needed for plant growth. The minimum threshold for lettuce is 4°C, potatoes 7°C, and for corn, 10°C. A killing frost is defined as -­‐2°Celsius or below. The median date for the last killing frost at the airport is May 13, but it can be as late as June 13. Generally speaking, eastern Calgary has a longer growing season than western Calgary.
  • The highest point within the city is at 1295 m and the lowest is 975m along the Bow River, a difference of 320m or 1050 feet, so there is a general northwest to southeast increase in temperatures due to the lower Equally important are the variations between different areas of your community … and within your own yard.
  • As our average precipitation is 420 mm, we live in a short grassland biome … so many of the trees and plants we grow will not survive without extra water and extra

2: Winter protection

  • Most Calgary gardens need winter protection, especially because of our Put organic material such as compost, shredded leaves or well-­‐rotted manure around the perennials and shrubs after cleaning up the foliage and weeds. This mulch will protect the perennials and retain soil moisture during the winter. When using leaves as mulch, consider leaving them on the beds in the spring -­‐ this organic matter, just like compost and manure, will decompose and add nutrients to the soil. One goal of winter mulch is to keep plants dormant through the winter, so it should be applied after the ground is cold, so the timing of application will vary from year to year. Deciduous leaves are effective insulators: delicate split-­‐leaf weeping birch leaves are ideal; oak leaves tend to stay dry and decompose slowly, and the larger poplar leaves are best shredded. Wood chips and bark, and conifer branches are useful, but straw may encourage mice, may seed itself and straw is time-­‐consuming to remove in spring. To provide winter protection, apply a 2 to 4 inch layer of shredded leaves around perennials (except in Alpine rock gardens).
  • The sequence is – clean up, weed removal , prune, plant bulbs, then mulch.
  • Collect leaves for compost as well -­‐ mix with grass The smaller the pieces, the faster they’ll break down, so shred or chop dry leaves before adding them to the compost pile. If you don’t have green trimmings or grass, add a source of nitrogen to the leaves, such as commercial fertilizer or cow, horse, sheep or poultry manure.
  • Watering: after freeze-­‐up on sunny days during the winter months, warm Chinook air speeds up transpiration
  • … loss of water … but the roots may still be Desiccation can cause the death of a plant but this can be reduced by watering regularly and abundantly until freeze-­‐up (once every seven to ten days), and occasionally in winter spray water on foliage during Chinooks. An anti-­‐desiccant can be sprayed carefully on evergreen foliage and rose stems.

3: Perennials

  • Remove damaged stalks and diseased Retain “architectural” plants such as Liatris, but trim the rest down to about 6 inches. Enjoy the winter hoar frosts and snows on the dead stalks: leave seedpods and flower heads that will provide winter interest or food for the birds. These can be cut down in April. Examples of these are Astilbes, grasses, Sedums, Rudbeckia (black­‐eyed Susan), and Echinacea purpurea (purple cone flower).
  • Do not put diseased plant remains (e.g. parts affected with powdery mildew) in compost Remove leaves if blackened, flattened, or destroyed by frost -­‐ this happens to Hosta, Daylily, and Peony.
  • Remove any weeds and slugs that were hidden by the perennials
  • Remove seed heads from prolific seeders such as Mullein and Daisies, unless you like the spires as winter
  • September and May are both good months to split
  • Trim mat-­‐forming plants such as
  • Begin planting bulbs in Daffodils should be planted before Sept 15, and small bulbs should be planted before the end of September. Tulips can be planted as long as the ground can still be worked, but ideally by October 1. Plant in groups or clumps, and plant larger bulbs in uneven numbers for a more natural look … depth related to bulb width. Use blood meal to discourage squirrels.
  • Some plants, such as perennial (and annual) ornamental grasses, provide winter interest in their dried In areas where the soil is prone to erosion by wind or water, such as on a slope, leaving the dead stems throughout the winter can help hold soil in place.

4: Annuals, including vegetables

  • Harvest Wrap green tomatoes in newspaper to ripen them or use them for specific recipes. Carrots can be left in the ground in cold weather perhaps up to Christmas time, parsnips too.
  •  Remove and compost annuals and vegetables if bitten by frost but not diseased. Spring bulbs can be planted where you normally plant annuals.
  • Some annuals can be over­‐wintered Take cuttings for houseplants from geraniums, impatiens, and fibrous begonias; rosemary too may be overwintered indoors.
  • Lift and store tender “bulbs” such as dahlias, tuberous begonias, and
  • Prepare the ground for future vegetables with cover crops (green manure), manure, and other Collect leaf mold for “carbon sequestration”.

5: Trees and Shrubs

  • Pruning of trees and shrubs (except spring flowering shrubs) is generally best done after the leaves have fallen, except for birches and maples, which will bleed if pruned in Dead or damaged limbs should be removed as soon as identified. Check legal pruning times for Elm with the City of Calgary web site: their pruning should only be done during the dormant season when beetles producing Dutch Elm Disease are not active, (ie from October to March).
  • Question your need for tender trees if they need to be wrapped against winter sun and wind!
  • Always remove and discard rose leaves with black spot Stop deadheading by August 15th. Pruning of roses, especially hardy roses, can be left until after they have started to leaf out in the spring. Shrub roses can be cut back to about 3 feet, but winter kill can still occur requiring a final spring prune. Add 6” to 8” of mulch or light soil around the base of tender roses in October.

6: Lawns

  • Repair bare spots with seed or Re-­‐seeding is best done in early or mid-­‐September, (or early spring). Keep newly seeded areas moist.
  • After August, turf grasses and other plants need a slow growth period to harden off for winter, so use low nitrogen, high phosphate fertilizers in late
  • Continue to mow the lawn until growth
  • Rake leaves off the lawn for mulch or compost before the snow Leaves left on the lawn and long grass will encourage snow mold. By mid-­‐September, the height of your lawn grass should be about 2 to 2.5 inches.

7: Miscellaneous

  • Improve the soil of your vegetable or annual beds by adding organic content; – when you add the winter mulch of compost, leaves or manure, you are feeding your soil. Warning: fine peat moss can blow away, and it adds very little nutrient Opinions vary on the desirability of turning over the soil. Much of Calgary has clay­‐rich soil: bury (or remove) the clay!
  • Mushroom waste can also be used, but sparingly. If you can wait, mushroom waste is usually free from local growers on Mother’s Day. Google “spent mushroom waste substrate” to learn more.
  • Prepare new beds, remove all weeds and work in organic material.  If you don’t mind the appearance, you can use the lasagna method to clear weeds or grass from an area you intend to improve.
  • Store clay and ceramic pots inside or covered or turned upside down: strawberry pots with Mexican Hair Grass and Hens & Chicks, may be kept outside if you can accept possible frost.
  • Drain hoses and store inside: turn off outside water sources from within your
  • Some containers and window boxes left in place can be used for Christmas
  • Apply any finished compost to the garden but also save some for the Leave some finished compost in the pile as an accelerator if you plan to continue adding equal quantities of shredded brown and green feedstock during the winter. Aerate the pile. Expect the composting process to slow down during the winter when cold-­‐ resistant bacteria and some microorganisms will continue to break down organic waste. The pile can be insulated with straw bales and discarded carpeting to help it retain heat. A thick layer of snow will likewise insulate the pile, although a combination of food and warmth could attract mice to nest in the pile over the winter. Fall leaves can be added in spring.
  • Clean, sharpen and oil all garden Maintaining the lawn mower and hedge clippers may be left to the professionals. Paint handles bright in colours for better visibility if left submerged in foliage.
  • Decide where to place bird feeders to create the least damage to surroundings. Find, clean, and install bird­‐bath heaters to provide birds with winter water.
  • Fall is a good time to fence your garden.  Deer need at least a six-­‐foot fence, with another foot or more of wires above that. How about trellises for grape vines, hardy kiwis, and so forth? If these are in your plans, this is a good time to get them built.
  • Assess your garden’s design … check for beautiful Fall colours … photograph different views (throughout the year), journal your ideas.
  • Relax and celebrate the year – prepare to give Thanks!


The authors are very appreciative of our two principal source references:

Toronto Master Gardeners in association with the Toronto Botanical Garden http://www.torontobotanicalgarden.ca/mastergardener/FallGardentoBed.shtml

Rosie Lerner, Extension Consumer Horticulturist, Purdue University. http://www.hort.purdue.edu/ext/gardentobed.html

The authors can provide their other references: contact us through the Calgary Horticultural Society.