Hi not sure when to uncover dwarf alberta spruce trees. Also city line hydrangeas.
Thank you for contacting the Toronto Master Gardeners. The gardeners rule of thumb is that uncovering trees and shrubs should be done after the last frost date. Below is a link to a map of Ontario that shows average last frost dates. Keep in mind that these are average dates, based on historical data, so they are estimates only. There is a 30% chance that the last frost will be earlier or later that these dates.
It’s best to pay close attention to your local weather forecasts to help you to decide exactly when to uncover your plants. The spring is notorious for weather swings, and plants are most vulnerable to winter injury as they are coming out of dormancy.
Hopefully this is helpful guidance for you.