Pot perennials in outdoor containers

Blooming pots are right at home with every garden style and every lifestyle. Most of us fill them with tried-and-true annuals, but garden consultant Michael Pascoe, professor of landscape design at Fanshaw College in London, Ontario, likes to experiment with perennials.

“Many live happily in containers,” says Pascoe. “As Canadian gardeners become more familiar with perennial beds, moving perennials into pots is a logical progression.”

Whether your garden grows 20 storeys high or spreads out over 20 acres, container gardening has undeniable appeal. When Pascoe was given carte blanche to design the 20-acres of lush garden for Mac and Patricia Cuddy near Strathroy, Ontario, he made liberal use of pots of all sizes — from four inches in diameter to seven gallons in volume — filling them with a profusion of perennials and some annuals.

Random groupings
The Cuddy garden consists of seven distinct areas, including a rose garden, a large herbaceous border, a water garden and an English perennial border. Containers provide punctuation marks in each of them, especially on the patio and at the entrances.

Pascoe likes to group the containers randomly, moving some of them around throughout the year. Others provide seasonal accents. When it comes to choosing between single or multiple varieties for one pot, he says, “It depends on the plants. An ornamental grass such as Miscanthus sinensis ‘Morning Light’ is a consistent performer with good form through the season and looks great on its own. But by August, Rudbeckia ‘Cornflower,’ for example, shows too much bare leg and could do with the help of a showy little mate such as Heuchera ‘Coral Bells.'”

Pascoe offers the following advice for would-be perennial potters:

Pick the pot
Look for some relationship between the container and the landscape-colour or texture, for example. At the top of the list is good-quality terra cotta. Because it’s porous, it becomes attractively patterned with salt and algae on its exterior as it ages.

Frost-resistant terra cotta pots can winter outdoors planted with perennial grasses or filled with evergreens, dogwood or holly. Also, use stone or concrete with a simple design.  Perennials need more room than annuals, so “think big,” he advises.

The dirt on soil
Garden-variety soil won’t do. Instead, choose a commercial mix such as Pro-Mix BX, a blend of peat moss, perlite and vermiculite with a good ratio of pore space to particles. The ratio allows the mix to absorb water quickly but not hold so much that the plants become waterlogged.

Choose the plants
Select a colour theme with contrast in both foliage and bloom. Pascoe likes subtle combinations such as the silvery-blue of lambs’ ears (Stachys byzantina) and blue oat grass (Helichtotrichen sempervirens) with a hit of bright colour delivered by hot pink campion (Lychnis chalcedonica).

Use lots of loose forms-grasses such as Japanese blood grass (Imperata cylindrica)-but  structured, rigid plants such as Verbascum mullein sparingly.

When buying a perennial, check for a solid root mass – what lies beneath is more important than what grows on top. Tip the pot and slide it off the root ball to see that it is well rooted.

For windy balcony gardens, which pose a greater challenge, choose drought-resistant plants-recognizable by its silver-blue foliage – and use large pots. A winning combination: Perovskia ‘Russian Sage’ combined with drought-tolerant annuals such as Echeveria.

Food and water
Most people water incorrectly. Too little can quickly prove fatal. Too much can make plants sickly. You must be religious in their care. Pascoe’s four commandments for care are:

1. During the active growing season, you will probably need to water daily, but let the pots dry completely – to the point where the plants just begin to wilt.

2. Toxic levels of chlorine build up quickly in containers. If using chlorinated tap water, fill the watering can and let it sit for 24 hours before watering to allow most of the chlorine to evaporate.

3. Water gently until water runs out of the bottom of the container.

4. Fertilize for three weeks after planting with a solution of 10-52-10 (1/2 tbsp./gallon) replacing a normal watering. Then fertilize weekly with a 20-20-20 solution (1/2 tbsp./gallon).

Extend the season
Since you will probably pour more money into planters than a garden bed of similar size, it pays to extend the season by covering the containers with a sheet on chilly fall nights.

Some perennials such as loosestrife (Lysimachia) (not the invasive kind), can be cut back and forced to produce a second show without detracting from the appearance of the planter.

Transfer potted perennials to the garden at the end of the season, or overwinter them in their pots in a protected area.