Planting After the May Long Weekend: A Zone 5 and 6 Guide

Every gardener in Ontario knows the rule: don't plant until the May long weekend. Like most old rules it's half right. It matters a lot for some plants and not at all for others, and knowing the difference gets you weeks of extra growing season.

Where the rule comes from

Most of the GTA sits in hardiness zones 5b to 6b, generally milder as you get close to the lake and cooler as you climb toward Caledon and the Simcoe area. Around the city, the last spring frost typically lands somewhere in the first half of May, with inland areas running a week or two later.

The long weekend rule is really a frost rule. By two-four weekend, the odds of a killing frost have dropped to nearly nothing across the region. So anything that dies at zero degrees waits for it. Anything that shrugs at frost never needed to wait.

What could have gone in weeks ago

Trees, shrubs and perennials are frost-hardy by definition, they were outside all winter at the nursery. We plant these from the moment the ground can be worked in April right through fall. Early planting actually helps them, since roots establish in cool moist soil with no heat stress.

Same goes for cool season vegetables like lettuce, peas and onions, and the tough annuals like pansies. April is their season, not a risk.

What genuinely waits for the long weekend

The frost-tender crowd: tomatoes, peppers, basil, cucumbers, squash, and most of the annuals that fill patio containers, impatiens, begonias, coleus and friends. One cold night flattens all of them.

And here's the part the rule leaves out: even after frost risk is gone, the warm-lovers want warm soil. Tomatoes and basil planted into cold clay in mid May just sit there sulking. If nights are still dropping below 10 degrees, waiting another week costs you nothing, the June-planted ones catch up and pass the shivering early birds almost every year.

Harden off, or pay for it

Greenhouse-raised plants have never felt direct sun or wind. Move them straight from a garden centre shelf to a full-sun bed and the leaves scald white within days. Give new arrivals a transition week: a few hours outside in gentle light, working up to full days, before they go in the ground. It's the least glamorous advice in gardening and it prevents the most common May casualty we see.

Planting so it takes

However good the plant, the first ten minutes decide a lot:

• Dig wide, not deep. Twice the width of the pot, no deeper than the root ball. Plants sink in fluffed-up deep holes and end up buried.
• Loosen circling roots. A pot-bound root ball keeps circling underground for years if you don't tease it apart. Be rougher than feels polite.
• Set it at grade. The top of the root ball should sit level with the surrounding soil. The number one planting error we fix is trees and shrubs planted too deep.
• Water it in properly. A slow full soaking the day it goes in, no exceptions, even if rain is forecast.
• Mulch after. Five centimetres over the root zone, kept off the stems.

Then comes the part that actually determines survival: the first summer of watering. That's a whole post of its own, coming next month, because June heat arrives faster than people expect.

If your planting plans outgrew your weekend, our crews plant all season, and everything we install goes in the ground the right way the first time.