For the best choice of annuals, it is ideal to be at the flower farm and purchase as soon as possible in the spring. However, most years the weather is not yet reliably warm, even by Victoria Day weekend. It’s safer to harden off your new annual plants before you put them directly outside.
If you wait until later, there are no guarantees that you will get the plants you wanted! It’s a catch 22!
This is what I suggest
Buy now and harden off your annuals at home. Put your new hanging baskets, planters or individual pots or trays of plants outside in a sheltered location in the day and if the temperature is forecast to drop below 5°C, move them back inside the house or in a shed at night. Do this for a week or two or until you consider it safe to plant.
This method is called ‘hardening off’ and it gets the plants acclimatized to being outdoors gradually. That way, when you do plant them they will be more accustomed to wind, rain, cold etc. Even if you do decide to take the plunge and plant early and we don’t get frost, the ground will not yet be warmed up enough for any growth to take place. The plants may not die due to frost, but they will just sit there waiting for some heat before they start to root in and grow. Of course, violas, perennials and shrubs are fine to plant anytime and are not affected by frost.