Term Definitions:

• Hardiness Zone (“Zone”): The US Dept of Agriculture breaks the US into “Hardiness Zones”. Hardiness Zones are a descending scale of the lowest average temperature for an area in a given year (which also roughly correlates with the zones average highs). This helps define what plants are likely to thrive in that area.

• Zones can be listed as the whole number (ie. Zone 6 or 9) or at one of the 2 (a/b) sub zone levels (ie. Zone 6a or 9b). If you see the whole number, that means the plant is fine both subzone a and b.

• Zones are defined down to the street level, you can find your zone here:

https://planthardiness.ars.usda.gov/

A common issue for any potted plant owner is what to do with outdoor potted plants once the weather turns cold. Though it may seem obvious to just leave their pots where they stood over the warmer months, doing this will risk you plant’s roots freezing (or simply getting too cold) and thus their untimely demise.

The goal of safely overwintering potted plants is to prevent 2 things:

1. Roots Freezing / getting below their hardiness temperature

2. Rapid freeze / thaw cycles (i.e. from the sun heating the pot during the day above 32° F, then it refreezing at night)

In ground, these 2 issues are prevented by the soil. The soil insulates them from getting too cold, and slows the freeze / thaw cycle enough that plants can naturally adjust. In the vast majority of the continental US, it is rare for in-ground soil to get below 29° F, so few plants have evolved for that level of cold on their roots.

Why this matters is that pots rarely have enough thermal capacity (heat storage by mass) to keep their soil not just above freezing, but from getting down to the much lower temperature of the air.

(On a side note, many colder zone species need freezing temperatures to properly germinate / grow, this is called cold stratification, which I will cover in another post.)

For plants to (probably) safely overwinter in their pots outside the rule of thumb is that they are comfortable down 2 Hardiness Zones from where they are located.

For Example: Highbush Blueberries are cold hardy down to Zone 4a. You can overwinter them outside in zones down to Zone 6a.

If the above rule of thumb doesn’t apply to the plants you have, you have several options:

1. Store in a fully enclosed, unheated (never gets above 55 F during winter) shed or garage, away from direct sunlight (to avoid the sun heating the pots during the day)

   1. If the shed/garage gets below freezing / below a plants hardiness zone:

      1. Wrap the pots in several inches of insulating material like bubble wrap, cardboard, or others (like straw. mulch, or burlap)

      2. Water periodically (plants still need water in the winter, just much less. The goal is to keep the soil from completely drying out, to slightly moist, depending on the species)

         
         
         
         

2. If you don’t have an enclosed space (or no room):

   1. Move the pots to the most wind protected spot (like against your house or solid fence)

   2. Elevate them off the ground to help with drainage and prevent heat loss to the soil

      1. I typically rest them on a bed of gravel, or pieces of scrap wood with a space open for the pot’s drain hole

   3. Cover the pots up to the pot’s soil level in insulating material. This can be done in multiple ways:

      1. Wrap the pots in several inches of insulating material like bubble wrap, straw, burlap, or others, and then a waterproof tarp. Put a thin layer of mulch on top ~.5 inch, but leave the stem / root flare exposed to air

      2. Bury them in a mulch pile, at least 6 inches thick on all sides, with ~.5 inch on top (leave the stem / root flare exposed to air

      3. Dig a trench slightly deeper than needed for the plant to be at ground level (as if the plant were planted in-ground). Fill the bottom with gravel (hence the slightly deeper depth), put the pot in, and backfill. Put a layer (.5 - 1 inch) of mulch over the top of the pot, leave the stem / root flare exposed to air.