There's nothing tricky about growing plants in containers as long as you offer

them good soil, food and water.  But because a plant growing in the ground is

more protected from severe cold (and alternate freezing and thawing) than one in

a container, the real challenge is overwintering those potted specimens so they can

adorn your landscape next spring.

     As with any game plan, you need to know your players. Some plants are easier

to overwinter than others, while some are more trouble than they're worth.  How

cold your winters are and to what lengths you're willing to go to protect your

containers are also factors.  Here are methods I recommend for saving your plants

for next season.

Hardy and borderline-hardy herbaceous perennials

     These plants usually die back and are dormant in winter, while their roots sleep

until it's time for new growth next spring.  Examples include hostas, Shasta

daisies, heuchera, astilbe, lady's mantle and daylilies.  The goal is to maintain

dormancy and provide a winter environment that's within their hardiness zone.

     After a couple of light frosts, water plants well and choose one of the following

storage options:  Leave as is.  If the pot is large and the plant is at least one

hardiness zone below your area (i.e., if you live in Zone 5, herbaceous perennials

in containers need to be hardy to Zone 4 or lower), the likelihood of successfully

overwintering the pot outdoors is high.  A large container holds more soil, which

helps insulate roots and keeps soil temperatures consistent.  However, when sun

hits the sides of a container, especially a dark-coloured one, alternate freezing and

thawing may trick the plant into thinking it's spring and trigger early growth,

when it's merely a warm day in February.

     Store borderline-hardy plants or those in small containers in an unheated

garage or shed.  Because the plants are dormant, light isn't required, but check

periodically to make sure the soil isn't bone-dry.  When growth resumes in late

winter/early spring, reintroduce plants to normal growing conditions outdoors

by gradually exposing containers to the elements for increasing periods of time.

     Find an area, such as the vegetable garden, where you can sink the pot in the

ground up to its rim so roots will be better insulated.  Cover the entire plant with

about five centimetres of winter mulch, such as shredded bark or leaves.  In spring,

remove mulch and lift out your container.

Hardy and borderline-hardy deciduous shrubs, small trees and

vines

     The stems, branches and trunks of these plants persist over winter, but the

leaves drop off in the fall.  Examples include miniature roses, rose standards,

Japanese maples, clematis and dwarf lilacs.  The goal is to keep plants dormant

and within the range of winter temperatures they would tolerate if grown in the

garden.

     Use large containers and plants that are at least two zones hardier than

recommended for your area.  These woody plants have above-ground branches

that hold next year's flower and leaf buds, making them more vulnerable to winter

winds than herbaceous perennials.

     For the best protection, store them in an unheated garage, against the warmest

wall.  If practical, place them in a garbage bag loosely filled with dry leaves for even

more insulation (leave the top open for air circulation).  Keep in a dark part of the

garage; light may trigger growth too early.  Once a month, check soil to make sure

it's not bone-dry, but do not overwater, as this could cause plants to rot or break

dormancy.

Hardy broadleafed and needle evergreens

     These types of plants transpire (lose water) during winter, and when

temperatures remain below freezing for long periods, root balls freeze solid and

water is unavailable to the plant, causing leaf damage or possibly death.  Examples

include English holly, boxwoods, English ivy, cedars, junipers and yews.  The goal

is to keep them within their hardiness zone and prevent desiccation from winter

winds.

     Move pots to an area where they will be protected from strong winter sun and

winds, and erect a burlap screen around them (don't allow burlap to rest on

foliage).  Keep plants well watered until freeze-up and check frequently throughout

the winter to make sure soil is moist.  Thorough watering prior to freezing

temperatures and again in March and April, when the root balls are most prone to

thawing-and drying out-is crucial.

Tip

     Don't forget to assess the winter hardiness of your pots.  Soil-filled containers

exposed to long-term freezing may expand and crack.  Those made of earthenware,

ceramic and terracotta (unless designated as frost-resistant) are especially

vulnerable; concrete, wood, plastic and metal are more durable, but the last two

materials offer little insulation for plant roots.

Tender evergreens and tropicals

     These plants don't require a period of dormancy.  Examples include bay tree,

rosemary, French tarragon, phormium, mandevilla, passionflower and jasmine.

The goal is to slow the growth of these plants so they can survive indoors during

the winter and be put outside again once all danger of frost has passed.

     A few weeks before fall frosts are expected, gradually introduce plants to life

indoors by moving them inside for half a day, working up to a full day over a

eek or so.  Don't wait until night time temperatures are only slightly above

freezing-plants may go into shock and drop leaves.  Lightly prune, if desired.

Place them in the sunniest location you have and water sparingly but regularly

over the winter; high humidity helps, too.  Don't fertilize until just before you

reintroduce them to outdoor living.

Tender tropicals that grow from tubers, corms or bulbs

     Examples include cannas, caladiums and elephant's ears.  The goal is to

store the tubers so they neither rot from too much moisture nor dry out

completely and die.  When frost kills back the tops of the plants, trim the stems

to 10 to 15 centimetres from the base and gently dig up the tubers.  Allow the

tubers to dry for a day or two, then store them in vermiculite or dry peat moss in

a crate or box; place in a cool, dark area, such as a basement.  (Check websites or

gardening books for more detailed instructions for specific kinds of tubers, corms

and bulbs.)

Tender perennials most often grown as annuals in cold climates

     First consider whether it's worth your time and effort to maintain these plants

over the winter; most are easily acquired at a reasonable cost in the spring.

Examples include geraniums, coleus and impatiens.  The goal is to continue

growing these plants to use them again next year.

     Trim back individually potted plants and place on a cool, bright windowsill or

under grow lights.  Alternatively, take cuttings of your favourites and start new

plants indoors.  By spring, they will be large enough for containers.

High-rise help

     Balconies can be especially brutal for potted plants.  Your best hope is to use

large containers insulated on the inside with thick pieces of Styrofoam or fasten

pieces on the outside during winter (bungee cords are good for this).  If possible,

move pots to a sheltered spot, out of wind and sun.  Cluster them together with

the hardiest plants on the outside and the most tender ones in the centre.  For

extra protection, wrap the huddled pots in a large tarp, but leave the top open

for air circulation.

Article provided by Holland Park Garden Gallery.