In Ontario, the best time of year for landscape installation is in the spring after all of the snow and ice have completely melted. Keep in mind that certain parts of Ontario can sometimes still experience freezing rain, snow, and hailstorms throughout the month of April and even in the beginning of May.
A good rule of thumb to follow is to wait until the May 2-4 or Victoria Day long weekend. Usually, the climate warms up sufficiently by that time. Planting flowers and greenery during the spring gives your plants ample time to adjust to their new environment during the summer and autumn months. Alternatively, you can also do some minor landscaping in the fall before the first frost.