Container questions answered

Mark (right) and Ben (left) Cullen pose for a photo with some of their plants in containers.

Everyone, it seems, is planting in containers. Condo owners create colour and grow food on their balconies while “home” owners do the same on the patio, front porch, and virtually everywhere that a pot or window box fits the bill.
What pitfalls are to be avoided? How can you get the best performance from your container grown plants: the most colour, butterflies, or tomatoes?
These are some frequent questions, and here are our answers:Should I use last year’s soil? No. The nutrients in last year’s soil are used up, and the only thing that might be “living” in it are spores for various root-bound diseases which you do not want to carry over. Time to refresh and revitalize using a quality mix that is designed and blended for container use. Do not throw last year’s soil out, put it in a garden and if you don’t have one, give it to someone who does.
Clay or plastic pots? We prefer clay as it breathes, transpiring moisture through its porous surface in exchange for oxygen. All plants like this arrangement. But it does mean that you will water a little more frequently than you would with plastic.
How do I choose plants for my containers? Ask yourself, “what am I trying to achieve?” A potted wildlife garden containing bee balm, echinacea, and marigold will attract beneficial insect and butterflies. If you just want colour, look for plants that suit the sun and wind exposure of the container’s location. It sounds simple but it is surprising how often people get this wrong. If you are growing food, like a tomato plant, give it enough space and sunlight to perform. A tomato plant will need two bushels of soil to grow well, which is more than most people realize.
Is drainage important? Drainage is the #1 consideration when choosing a container. There should be a minimum 2 cm hole for every square foot of soil surface. For any plant to perform well, water must move through the soil and out the bottom freely. Where it goes is another problem: onto the balcony below or your new outdoor carpet? We hope not. Best to obtain a saucer to catch the extra water.
How do I know when to water my container plants? While there is a wide variety of demands for water, depending on the plant type and species, our general rule is to push your index finger into the soil up to your second knuckle. If it is cool and damp, hold off, if it is dry, time for a good drink. Best to water thoroughly, less often, than sprinkle water every day. Note that most herbs perform well when allowed to dry between watering, as drying down minimizes the odds of root rot or fungal-borne disease.
Do I need to fertilize my container grown plants? Most quality container soil mixes contain slow-release fertilizer or enough organic content in the form of compost that no fertilizer is generally necessary until roughly mid-season. As plants mature they consume those nutrients, and as their roots eventually fill the container there is a greater need to fertilize. We recommend either an organic based liquid plant food or an application of water soluble 20-20-20 every week to 10 days to “top up” nutrients for best performance.
What do I do with my container plants when I go on vacation? If the containers are not too heavy, move them into a shaded spot and water them thoroughly just before you leave on vacation. If they are too heavy to move, just water them well before you go. Of course, the best course of action is to have a plant-sitter keep an eye on them. Our experience is that most amateur plant sitters tend to over water, so leave specific instructions to minimize the risk of returning home to rotten plants.
Growing in containers provides a great opportunity to maximize food, flower and greenery around your place. Go for it!
Mark Cullen is an expert gardener, author, broadcaster, tree advocate and Member of the Order of Canada. His son Ben is a fourth-generation urban gardener and graduate of University of Guelph and Dalhousie University in Halifax. Follow them at, @markcullengardening, and on Facebook.