Container gardening: Best time to transplant vegetables

Q. Last year I made a small attempt at growing mainly vegetables and a few flowers in containers. I still have questions: When is a good time to sow or transplant outside in containers? How cold is too cold? How do I prepare the soil for planting in containers still holding last year’s planting mix? In what sort of conditions should the containers be placed?

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A. The timing for planting outdoors will depend on what you are planting. Lettuce and other leafy greens, radishes, carrots and beets can be seeded now. Lettuce transplants are available. Seed for short-rooted carrots (suitable for container cultivation) should be available on local seed racks.

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Wait until late April or May, when overnight temperatures no long dip below 10 C, to plant warmth-loving plants like tomatoes, peppers, cucumbers, bush beans and zinnias. Look for compact varieties of flowers and vegetables to grow in containers, such as “patio” tomatoes, for which you’ll find transplants in garden centres.

There are compact, container-friendly varieties of most vegetables these days — cucumber, zucchini, beans, peas and more.

Heat-loving plants will be fine in full sun as long as the pots are large enough to hold an adequate amount of heat-buffering soil. Commonly grown salad vegetables will benefit from light shade in hot, sunny weather.

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When reusing containers still holding last year’s planting mix, it’s helpful to remove around a third of the old soil, loosening what remains in the container and topping it with a fresh soil mix.

I use the removed soil to mulch garden beds. If you have no garden space for this, perhaps offer it to a friend with a garden.

Many commonly used planting mixes are rather lightweight. To lend heft and “staying power” to them I search for bags of heavier types of potting soil for mixing into them.

A little coconut fibre (coir) is a useful element in a mix. Coir enhances moisture retention and acts as a buffer against temperature extremes.