We do our best to send hardened off plants to our garden center customers and landscapers alike. However, if those plants end up in a greenhouse, they will no longer be hardened off in short order!
Plants that have been living indoors need to be acclimated to all the temperature ups and downs, the wind, and the sunlight of living outdoors. They need to be hardened off.
Hardening off methods are typically used from about mid-March to late-May here in Wisconsin.
The process of hardening off may take about a week, and helps plants move from a controlled indoor environment that encourages soft foliage and lanky stems to withstand the rigors of changing outdoor conditions. Traditionally, it involves taking flats or pots in and out of the house, gradually increasing the amount of time they are outside. As plants spend more time outside, their stems get stronger and their leaves develop a thicker cuticle, which is the waxy covering on a leaves surface that prevents water loss.
- Start by finding a place in a protected spot out of direct sunlight and strong wind, like under a tree, beside a hedge or near a wall of the house or garage. Bring out the plants to be hardened off for an hour or two in the mid-afternoon (or whenever the temps are closest to your interior temps) Bring them in for the night. A cart works great for this.
- Each day or two, increase the time spent outside by 1 – 2 hours. Watch the weather. If there are a few days of frost, a cold snap or driving rain, keep them inside.
- When the plants are used to being outdoors all day and the temps outside are mild, put them in a protected spot for the night.
- Full-sun plants need time to get used to direct sunlight as well. Start them out in part to full shade on the north side of your house or under a tree. Every couple of days move them into more sunlight until they are in full sun.
- Once plants stay outside day and night for 3-4 days in a row, they are ready to go in the ground.
Another method is to build a cold frame (temporary or permanent). Position your cold frame in full sun, near a water source and out of the cold north wind. A spot on the south side of the house or garage provides lots of sunlight and reflected heat. Set a thermometer inside (out of direct sunlight for an accurate reading) so you know what the temperature is.
Here is an excellent article on how to build and use cold frames: https://www.gardengatemagazine.com/articles/how-to/plant/diy-cold-frames/
Most Perennials love cool weather. They easily tolerate temperatures in the 40° F range and can be planted about a month before our likely last frost, which is around Memorial Day here in Wisconsin.