First things first: DON’T PANIC. Don’t whip yourself into a frenzy trying to cover every plant in your garden this afternoon. Instead, focus your efforts in a few targeted ways to protect the plants that will benefit from them the most. Here’s our list of cold-prep priorities:
PRE-FREEZE
Cover or shelter container plants.
Anything planted in a pot above ground is more vulnerable to the effects of frost because the roots aren't insulated underground. Take the following measures to protect your container gardens:
1. Put any containers, window boxes, or hanging baskets that are light enough to be moved into a garage or carport, or, at the very least, tucked close to the foundation of the house or under the eaves. This will protect against heavy frost and desiccating winds.
2. Cover container gardens with frost cloth that goes all the way to the ground. Secure the cloth with bricks or heavy-duty landscape staples so it doesn't blow off. The frost cloth must go all the way to the ground in order to protect the exposed roots of the plants. The frost cloth will also trap any residual heat rising off the sun-warmed ground further buffering the roots of the plants against the extreme overnight cold.
3. If you don't have enough frost cloth for all your containers, mound up fallen leaves, mulch, or straw around the base of the pots. Anything you can do to provide a layer of insulation between the roots and the cold air will help.
4. The same protection measures that apply to container gardens apply to plants that are still in their nursery pots. Group them close together in a sheltered spot and cover with frost cloth or mulch. Tip evergreen plants on their sides before covering in order to protect their leaves from the wind. Resist the urge to bring these plants indoors. Putting them in an unheated garage is fine, but they really don't want to spend the winter in your living room next to a roaring fire.
Cover agaves.
Succulents such as yucca, sempervivum (hens-and-chicks), and sedum (stonecrop) are very cold hardy and won't be harmed by the low temperatures. There are also a number of agave species that can handle the cold like champs. However, if you have some of the more tender species such as Agave americana (Century Plant), you should definitely provide protection. Use frost cloth, bubble wrap, a plastic tarp, or even a big cardboard box to completely cover the agave from its spines all the way to the ground. Remember to anchor the covering with something heavy to keep it from blowing off.
Unsure if the species of agave you have is cold tolerant? Cover it. Even the really cold hardy varieties will appreciate a little extra protection from the rain and extreme temps.
Mulch or cover newly planted perennials.
Newly planted herbaceous perennials - this means perennials that die back to the ground or to a small cluster of leaves at the crown of the plant - will need some extra protection in an extreme cold snap like the one we're about to experience. The best protection is mulch. Use nuggets, shredded bark, pine straw, or fallen leaves. Apply a thick layer to all the plants. If you're in a hurry, you can even mound it over the basal leaves at the crown of the plant as long as you remember to rake it back once the temps warm up again. The important thing is to get the shallow roots of the newly planted perennials insulated from the cold.
If you can't mulch, or for plants that are in a rock garden or an area where organic mulch isn't used, cover any newly planted perennials with frost cloth, making sure to anchor the cloth so it doesn't blow off.
Woody plants (this means trees, shrubs, conifers, and any perennials that grow thick, woody stems) will be fine during this cold snap. Make sure they have a good layer of mulch, and they'll be A-okay.
Water.
Watering deeply before a freeze is beneficial in two ways: it provides valuable moisture that the plants won't have access to should the ground freeze solid for an extended period; and, through evaporation, it helps raise the ambient air temperature immediately surrounding the plant. This rule applies to plants in the ground and in pots. The only exception is succulents. Keep them as dry as possible during cold weather, and protect them from snow and ice with a heavy cardboard box or other covering.
Use your best judgement when it comes to watering. If large amounts of rain are expected, you probably won’t need to supplement water unless you have container gardens that aren't exposed to the rain.
Drain hoses and outdoor spigots.
Remember to unhook and drain your garden hoses so they don't freeze and burst. Drain your outdoor irrigation lines and leave exposed spigots dripping so the pipes don't freeze.
POST-FREEZE
Remove frost cloth if temps rise.
It's okay to leave it in place for days (even weeks) as long as the temperatures don't rise above 50. You should remove the cloth once temperatures rise into the 50s so that plants can remain acclimated to the cold and not get too comfortable in their makeshift greenhouses.
Prune with caution.
Resist the urge to prune plants that are frozen. Allow plants to thaw before you begin assessing any damage. It’s also a good rule of thumb to wait until April to being pruning off any cold-damaged parts.