The diverse geography and variable climate of Ontario require precise timing for flower planting. Successful gardening requires understanding the local environment and matching a flower’s cold tolerance to the specific spring and fall weather windows. This guide provides practical advice for optimizing flower planting across Ontario’s unique conditions.
Understanding Ontario’s Climate and Frost Dates
The most important factor governing when to plant in Ontario is the date of the last expected spring frost. This date is the primary constraint for all tender plant varieties, marking the transition to the active growing season. Ontario’s climate zones range from Canadian Hardiness Zone 0 to 7, determined by minimum winter temperature, frost-free periods, and maximum summer temperatures.
Southern Ontario often sees its last frost between late April and mid-May, with many gardeners using the Victoria Day long weekend in late May as a safe benchmark. Moving north, the last frost date shifts later; Central Ontario may not be frost-free until mid-June, and some far Northern regions experience frost risk well into June. Checking the specific regional hardiness zone and the average last frost date helps gardeners select appropriate species and prevent early crop loss.
Timing for Annuals and Warm-Weather Flowers
Annuals and warm-weather flowers (such as impatiens, petunias, and begonias) are highly sensitive to cold. They must be planted only after the complete risk of frost has passed, as planting too early results in damage or death. For Southern Ontario, this means planting late in May or early June, ensuring both frost-free nights and sufficiently warmed soil.
Gardeners in Northern Ontario must typically wait until mid-June to safely transplant their annuals. Before planting out, any annuals or seedlings started indoors require “hardening off.” This is a gradual, two-week acclimatization period where plants are slowly introduced to outdoor conditions like direct sun, wind, and cooler temperatures. This process involves placing plants in a shaded spot for a few hours initially and progressively increasing their exposure daily. Hardening off prevents transplant shock and sun scald once they are permanently moved to the garden bed.
Planting Perennials and Hardy Species
Perennials and hardy species tolerate cooler soil and air temperatures, offering more flexibility in planting. The primary window for establishing them is early spring, as soon as the soil is workable (as early as April or early May in the south). Planting early allows the plant to focus energy on developing a strong root system before summer heat arrives.
A second, highly effective planting window for perennials is late summer to early fall, typically from mid-September through October, as long as the soil remains workable. Cooler air temperatures and increased moisture from autumn rains reduce heat stress on the foliage. The plant’s energy is redirected toward root growth, allowing the root system to establish itself before the ground freezes solid. Planting in the fall primes the perennial to emerge earlier and more vigorously the following spring.
When to Plant Flower Bulbs (Spring and Fall Schedule)
Flower bulbs follow a distinct planting calendar based on whether they bloom in the spring or the summer. Spring-flowering bulbs, such as tulips, daffodils, and crocus, require a period of cold dormancy to initiate bloom, a process known as vernalization. These must be planted in the fall, generally from late September until the ground freezes, which is often mid-October to late November in Southern Ontario.
The ideal time to plant these fall bulbs is when nighttime temperatures consistently hover between 5°C and 10°C, a signal that the soil is cooling down but still soft enough to work. Planting too early can encourage premature sprouting, while planting too late risks the bulb not establishing roots before the ground becomes impenetrable. Conversely, summer-flowering bulbs, including dahlias, gladioli, and cannas, are tender and cannot survive the winter outdoors. These bulbs should be planted in the spring, following the same schedule as annuals—safely after the last spring frost date.