Your container plants frost vulnerable nature comes from their exposed roots. They have far less protection than plants growing in the ground. I lost a beautiful potted fig tree my first winter. The limited soil in your pots cannot buffer temperature swings like garden beds can.
Pot walls are the main problem for potted plant roots during frost. They conduct cold directly from every direction. In-ground plants have earth insulating their roots from above below and all sides. Potted plants sit exposed with only thin material between roots and freezing air.
The container insulation issue is basic science. Small amounts of soil cannot hold much heat. Your container cools off fast once the sun sets. I checked temps in my pots versus garden beds last fall. The pots dropped 10 degrees faster.
The thermal mass of the earth gives in-ground plants a huge advantage. Ground temps stay warmer than air during frost events. I tested with a soil thermometer one cold night. While air hit 28 degrees my garden bed soil stayed at 38 degrees. My deck pots measured just 30 degrees at root level.
Above-ground containers lose heat from every surface. The bottom sides and even drainage holes all let cold seep in. Raised planters and hanging baskets suffer most. Air circulates underneath them too which cools them faster.
Frost damage containers often shows up first in root death not leaf damage. Roots are more tender than stems and leaves. Dead roots mean the plant cannot take up water. You might see wilting days after a freeze even though leaves looked fine at first.
Your container plants need extra care. I wrap my pots in bubble wrap or burlap when frost comes. Moving them into a garage or shed helps a lot. Even cold sheds add a few degrees of warmth.
For plants too heavy to move I push them against my house foundation. The radiant heat from the building provides some warmth through the night. A south-facing wall works best since it soaks up sun during the day.
Group your potted plants on cold nights. This creates a safe zone of warmth. Plants on the outside shelter those in the middle. I push all my containers into one tight cluster when frost hits.
You need extra frost awareness for potted plants. Mark your calendar with first frost dates. Start protecting them two weeks before that date. Your containers need you since they cannot stay warm on their own.
Read the full article: Protecting Plants from Frost That Works
