The key to success with garage storage is that this area is attached to the house but not heated. The garage never goes below 32 degrees but tends to hover in the 40's and low 50's all winter. This keeps the plant dormant but the plants never freeze. Watering is needed, but not too much or too often.
You can overwinter potted perennials in an unheated shed or garage. Allow them to naturally go dormant in the fall before moving them inside. Then put them back outside once the weather starts warming back up in late winter or very early spring.
A heated location isn't always necessary. Moving plants to a space like a garden shed or growing plants in a garage will provide enough protection when it gets cold. However, when lows are near freezing, it's time for an insulated indoor location.
The first option is to place the pot in an unheated garage (or other unheated but protected spot). The protection from the building will be enough to keep the pot from freezing too hard and to protect it from freeze/thaw cycles. Wait as long as you can without risking the plant before placing it inside.
Don't leave your plants in an unheated garage or shed, though they should be fine in a room with temperatures ranging from 55 to 70 degrees Fahrenheit. And while your plants need the same light levels as they received outside, keep them away from drafty windows.
The easiest way to protect from a freeze is simply by covering plants with a sheet or a blanket. This acts like insulation, keeping warm air from the ground around the plant. The added warmth may be enough to keep a plant from freezing during a short cold snap.
From that experience, I've found the best frost protection for your outdoor plants is either free or cheap. Cardboard boxes and brown grocery sacks make perfect frost cover and at the end of the season can be recycled. I keep various boxes on the patio and when frost is forecast simply put one over the plant.
A space heater or solar lamps is all you need to keep the garage warm enough for plants to thrive. Keep in mind that the hotter the room is, the more light and water your plants will need. So you only need to keep the room above freezing for your plants to grow.
It is a good idea, though, to give the pots just a little bit of water every few weeks over winter. The goal is to keep the soil from going so bone-dry that the roots shrivel and die, but not so damp that you encourage new shoots to come up in the somewhat-warmer temperatures of your garage.
The plants can then spend the winter in an unheated basement, root cellar, unheated garage, or even a cool closet. Make sure the area is relatively dark (try enclosing the whole pot loosely inside a heavy black trash bag) and that the air temperature stays above freezing.
Some experts recommend bathing or soaking plants in a bucket of water with mild dishwashing soap before bringing them inside. If your plants are too big for the bucket treatment, spray them with water to remove outdoor dust and soft-bodied insects from the leaves.
On woody plants, the leaves will yellow and drop off. Keep the dormant plants dry, but not so dry that the soil becomes powdery. Check soil moisture often and if very dry, lightly water. In most situations, light watering will be needed every two to four weeks.
Dormant plants need less frequent watering than those in active growth. (If planting in a low desert region, a watering once every 2 weeks may be needed if day temperatures are hot.) Check under the mulch. If the soil feels dry, then water lightly.
When it comes to heating your garage, you do not need to heat it to a comfort level equal to the remainder of your home (often between 65°F – 68°F). Keeping your garage thermostat steady at a temperature between 40°F and 50°F will give you benefits to the three listed items above.
DO NOT store cardboard or any paper products in cold rooms. Metal or plastic containers are allowed. If some paper products (e.g. Kim wipes) are required, place the item in a closed plastic container between uses. Should visible mold be found on a paper product, discard the item immediately.
Insulate the Garage Walls
This huge surface area is capable of conducting energy, letting it escape from your garage and making temperatures inside plummet to uncomfortable levels. Treating your walls to some insulation can work wonders in raising the temperature, especially when combined with the presence of a heater.
Cover Plants – Protect plants from all but the hardest freeze (28°F for five hours) by covering them with sheets, towels, blankets, cardboard or a tarp. You can also invert baskets, coolers or any container with a solid bottom over plants. Cover plants before dark to trap warmer air.
When the weather forecaster warns of an overnight freeze, cover your plants with burlap, an old sheet or blanket, or clear plastic. Use stakes or another support to keep the covering from directly touching the plants.
Fabric coverings will prevent the freezing air from coming into direct contact with the moisture on the plant while also capturing the heat that is radiating from the ground. Bed sheets or comforters work best for covering large plants and shrubs.
Despite the fact your plants are dormant and brown, they should still be watered periodically. Plants that remain dehydrated in winter months often don't survive until spring. Not only does this create extra landscaping costs in warmer months, it can actually damage your plumbing.
When frost threatens, it's time to move many of your outside plants indoors. Many tender bulbs, annuals, herbs, and tropical plants will only survive the winter inside. Here's advice on which plants to bring indoors this fall and how to winterize plants and pots.
Just be sure not to leave the pot in the ground too long in the spring or the roots will start to grow out the drainage holes, anchoring the pot into the ground. You can overwinter them by moving the pots into a cold frame or unheated garage for the winter after the first hard frost.