All plants listed below are cold-tolerant to at least 2C/35F and lower, and after a brief hardening-off period, can be planted out in window-boxes and containers this week. To harden off your newly-purchased flowering plants before planting (since they may have just arrived from Southern Ontario or even Florida), put them outside for longer and longer periods over the span of three or four days, bringing them indoors at night.
Calendula (aka Pot Marigold)
Gorgeous sunny daisies available in lemon yelllow, gold and red tones, this compact plant is ideal for containers. Needs full sun and well-drained soil. Flowers are used in hand creams, etc., to soothe dry and irritated skin.
Dianthus (aka Annual Pinks)
Bright cheery fringed flowers in all shades of pink and red were once sacred to Artemis in Ancient Greece but became the poor girl's sachet in Victorian times. Heirloom varieties have a strong clove scent. Full sun and good drainage a must.
Diascia (aka Twinspur)
This frothy delicate South African native is related to the Snapdragon. It comes in pastel shades and jewel tones. Trailing varieties are best for containers. Does best in partial shade in hot weather.
Dusty Miller (Senecio cineraria)
One of the few cold-tolerant foliage plants, the silver leaves of this plant come in many forms from fern-like to oakleaf. Needs full sun and well-drained soil. Good accent in mixed containers.
Godetia (aka Farewell to Spring)
Cup-shaped blossoms in white, pink, red, or lilac give a summery effect long before July arrives. This plant is native to British Columbia. Stick with the shorter 25 cm./10 in. varieties in containers. Full sun or partial shade.
Iberis (Annual Candytuft)
A small, compact plant literally covered in white, red, lilac or pink blooms. Full sun. Often found in wildflower mixes, very easy to grow.
Lobelia
No other annual is as brightly blue as lobelia and the trailing forms beautifully soften the edges of any container. Also available in pale blue, lilac and white varieties. Does best in partial shade in hot weather but can take full sun in spring and early summer. Trim back plants by one-half once blooming slows down in midsummer, to encourage a second flush of blooms.
Nasturtiums
With saucy upturned flowers and lily-pad-like leaves, Nasturtiums come in yellow, red, orange and and every hue in between. Available in trailing, vine and bush forms. Tolerates lows of 2C/35F. Seeds need warmth to germinate but hardened-off transplants can be used outdoors in containers at this time of year. If frost threatens, bring them inside. Leaves, flowers & seeds are all edible and brighten up salads with their mild peppery taste.
Nemesia
Another South African native, Nemesia is a small bushy plant with orange, purple, white or red tubular flowers. Prefers cool temperatures and will not thrive once hot and humid weather arrives. Full sun or partial shade and moist, well-drained soil.
Osteospermum (Cape Daisy)
Also from South Africa, these large daisies come in white, cream, yellow, lilac, purple or pink shades and have dark blue eye-like centers. Place in full sun, keep well-watered, does not tolerate hard frosts. Low-growing varieties work best in containers.
Pansies & Violas
One of the best spring annuals, pansies, and their tiny fragrant cousins the violas, even bloom through the snow. For a quick show, plant them 3" apart in containers. The colour selection is almost infinite with bi-colour, ruffled and striated petals. Removing the old flowers and stems keeps them flowering through to July. Pinch back plants once hot weather arrives, continue watering and fertilizing, and they will bloom again in the fall.
Petunias
Once properly hardened off, Petunias can tolerate lows of 2C/35F. They are one of the most reliable annual flowers and come in a range of colours from pink through red, lavender, purple, peach, white, cream, and even yellow. Trailing varieties work especially well in containers. Keep fertilizing to encourage flowering all summer.
Phlox (Annual)
A common wildflower in Texas, phlox drummondii or annual phlox is available in compact forms that work well in containers. Prefers full sun and well-drained soil. Remove spent blossoms to extend flowering time. Although it prefers cool temperatures, more heat-tolerant varieties are becoming available. Also attracts butterflies.
Primulas (aka Annual Primroses)
A perennial often sold as a spring annual, primulas are frost-hardy shade lovers that will tolerate partial sun in cool weather. The tiny flowers, either white, lavender, pink, red or yellow, nestle in tiny clumps of puckered leaves. Flowers usually fade once the summer heats up but containerized primulas can be planted out in the garden in early fall to bloom again next spring.
Snapdragons
Shorter varieties create great vertical interest in containers. New trailing varieties are worth checking out. Available in a multitude of colours, they prefer a rich, well-drained soil and full sun. Remove dead flowers and continue fertilizing to keep them blooming all summer. They are tolerant of 2C/35F temperatures once hardened off.
Sweet Alyssum
Fragrant delicate white, lavender or pale pink flowers make excellent trailing plants in containers. Plant in full sun or partial shade in well-drained soil. Do not over-fertilize - this plant likes it lean. Make sure to smell these before you buy - the white varieties are often the most fragrant. A member of the mustard family, the edible flowers and leaves taste like a broccoli.
Fragrant delicate white, lavender or pale pink flowers make excellent trailing plants in containers. Plant in full sun or partial shade in well-drained soil. Do not over-fertilize - this plant likes it lean. Make sure to smell these before you buy - the white varieties are often the most fragrant. A member of the mustard family, the edible flowers and leaves taste like a broccoli.
Sweet Peas
The princesses of the spring garden, sweet peas are both fragrant and fussy. They prefer to be directly-seeded into moist, rich, organic soil as early in spring as possible. You will need to provide a trellis, a bamboo teepee or string for these vines to climb. Showy flowers usually appear 6 weeks after germination and on warm days will scent the air with a heady perfume. Constant feeding ensures continuous bloom. There are hundreds of varieties available but heat-tolerant versions are best for Ontario.