Winter survival of our precious perennials is always a major concern. Keeping a few things in mind will help them come though the cold season. Remember that our chief winter enemies on the Front Range are: soil dryness, drying winds, fluctuating temperatures, and “false springs”. Plants in containers are especially vulnerable. If you want to experiment with over-wintering perennials in containers, the bigger the container the better. Barrel-size containers can work if they are somewhat protected from our drying winds and temperature extremes. I would not consider trying this with clay pots, even large ones, because damp soil can expand and crack them when they freeze. Thick wooden containers, or “closed-cell foam” plastic containers do provide some measure of insulation during temperature fluctuations.
Soils with a large amount of air space; sandy/gravelly soil, or soil with an over-abundance of organic or moisture-retaining materials, can actually let cold air penetrate more deeply, thus damaging plant roots. Nursery plants that have been rooted in very light “soil” material are susceptible to cold penetration even if they have been planted (sunk into) your regular garden soil. Winter soil moisture is critical. If we have little or no snow cover, water every 3-4 weeks on warmer days that will allow water to penetrate before it freezes. Keep the (dead) topgrowth on perennials as much as possible in winter. If we do have snow, any remaining topgrowth will catch snow that will add to soil moisture when it melts. Mulching around perennials is extremely important. It helps to retain soil moisture and reduces soil temperature fluctuations. A layer of shredded bark, pine needles, or other insulating material 3” deep or more will help greatly. Avoid using fallen leaves, these can mat down and mold.
Putting your perennials “to bed” properly during their “hibernation” season will let you sleep easier too. Then you can relax, read your garden catalogs and anticipate our next real spring!
Posted by Jeffco Master Gardener Dave