Insulating Planters

Your perennials will thank you for insulating containers. Mine show this by coming back stronger each year, even after harsh Chicago winters. Not all perennials require insulating planters (See Astilbe and Rupturewort), but many do. There are many effective options including Ceramalite, Polyurethane, and lining an un-insulated container.

3 photos of planters: ceramalite, polyurethane, and fiberglass


I found Ceramalite planters in a garden store chain in Wisconsin. It seemed like the store had purchased a whole cargo container of them, perhaps from Australia. I should have bought more than two.

Although Ceramalite containers are expensive, they’re worth it. I’m not sure of what Ceramalite is made, but I don’t think it’s ceramic. Ceramalite has several advantages:

  • Raised off the Ground – allows for great drainage if you drill enough holes
  • Long Lasting – looks as good as new several years later
  • Light Weight – can easily be moved to any corner of the garden
  • Effective Insulation – Arborvitae and Weigela are outside all year long


The following year I went back to the same store in Wisconsin. No more Ceramalite. Instead, the store put their own brand on a series of Polyurethane planters.

The Polyurethane costs less and looked almost as good. After a couple of years, however, the powder coated paint surface started to chip away, revealing the styrofoam-like interior. Although not environmentally friendly, the fact that Styrofoam doesn’t easily degrade, makes it a durable container. Polyurethane has several advantages:

  • Reasonably Priced – doesn’t require a lot of money or time to set-up
  • Easy to Maintain – a touch of paint around the rim every other year does the job
  • Light Weight – can easily be moved to any corner of the garden
  • Effective Insulation – Blue Star, Heuchera, and 5 other perennials come back every year


Lining an Un-insulated Container

You don’t need to buy an insulated planter. Lining containers, whether they are intended as planters or not, can be just as effective. I’ve used plastic and fiberglass planters. You can choose any container that withstands the weather in your climate. If the temperature ever goes below freezing, you may want to avoid ceramics that can break apart.

Lining your own planters has several advantages.

  • Unlimited Choice – can make or choose any container of weatherproof material
  • Any Price Point – can choose free planters and liners or pick a more expensive one
  • Any Weight – can be moved as easily as the container you’ve chosen can be moved
  • Effective Insulation – Cedar, Boxwood, and Violets come back year after year

The lining itself can be made of any number of materials with insulating properties. Between mom and an expert at a local gardening store, I’ve gotten some great advice. Here are three materials that work.

  1. Board Insulation – works for flat-sided containers, but installation requires effort
  2. Spray Insulation – easy to install and works on containers of any shape
  3. Newspapers – best on the environment and wallet, but requires replacing periodically

photo of pink insulation board lining the inside of a fiberglass planter

Notes on Lining Containers

Cutting board insulation to the right size requires a little bit of work. First, measure the interior dimensions of the planter. Second, mark and cut the foam pieces. To cut the pieces cleanly it helps to have the right tool such as an electric knife. Finally, place the cut board insulation pieces into the sides of the planter.

I usually drill more drainage holes than planter manufacturers intend. When installing spray insulation, stick straws with temporarily taped openings in drainage holes to keep them from getting clogged. Later remove the straws or remove the tape and cut off protruding ends of the straws. Regardless of the liner chosen, only line the sides of containers. Like any planter, I fill the bottom with a couple of inches of lava rock, then add dirt. Lava rock is extremely porous. Nothing should be in the way of proper drainage.

How have you insulated planters?