Garden Plot: Don’t leave plants in pots outdoors over winter

October 11, 2019

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Mary Lou in Fairfax writes:

I have a strawberry plant, a rosemary plant and a lavender plant in individual containers. I don’t know if I should bring them inside while the weather is still nice, or wait until colder weather comes. What do you think?

I think that this is a great opportunity to remind WTOP readers and listeners that plants in pots typically do not survive winter outdoors, and that includes plants that do survive winter when they’re planted in the ground instead.

The reason is simple: 100% of a plant in a container is exposed to the worst of winter weather, while the roots of a plant in the ground are insulated from the worst weather.

Terra cotta and ceramic pots are also virtually guaranteed to crack if left outdoors with soil in them.

Potted strawberry care

Now, about Mary Lou’s strawberry plant.

I think that those strawberries need to be planted in the soil or a raised bed outdoors and not be brought inside.

Strawberries — like many other plants — need to be exposed to the change of seasons; and they are extremely cold-hardy, thriving outdoors in the winter far to our north and into Canada. And June berries — the kind you most often see in supermarkets — especially need those natural timing cues of day length and soil temperature.

Pick their permanent outdoor spot carefully, as a strawberry bed should live many years and spread out laterally over time.

Rosemary care

And what about Mary Lou’s rosemary plant?

I think that while rosemary probably would not survive winter outdoors in a pot, it just might thrive if planted in the ground in a protected spot. (Not in a low spot out in the open. Next to a wall would be ideal. Good drainage is also essential.)

The greater D.C. area is kind of a dividing line for these popular half-hardy Mediterranean perennials. Rosemary, for instance, would die over the winter in Pennsylvania, but would almost certainly survive down in the Carolinas. And in the heat sink of D.C. itself.

Otherwise it’s a real crap shoot, and rosemary is a somewhat difficult plant to keep happy indoors.

If you don’t mind the exercise, keep it outside in its pot most of the time, but bring it indoors if temperatures drop below freezing or ice storms are predicted

The confusing world of lavenders

And now, for Mary Lou’s lavender plant.

Lavender is a tricky plant. English lavender is the most cold resistant; it would do fine in the ground in Fairfax, Virginia. Spanish lavender is the least cold hardy and would almost certainly die over winter. French lavender is somewhere in between.

If you know you have the English or French type, plant it outside in the ground in a sheltered area that has spectacular drainage. Nothing kills lavender faster than wet feet.

If you choose to bring it inside for the winter as a houseplant, make sure that the “soil” in the pot contains no actual soil; you want a light loose potting mix. Give it bright light and do not over water!

Get ready to protect ‘summer bulbs’

  • If you are growing “summer bulbs” such as dahlias and tuberous begonias, be prepared to lift them out of the ground and store the roots, rhizomes, tubers or corms indoors. (None of them are true “bulbs” like tulips.)
  • Wait until right after the first light frost, carefully dig up the underground plant parts and put them in a cool dry area with lots of airflow to cure for a week or two. Then, put them in a container filled with peat moss and replant them outside next June.
  • Canna lilies — those dramatic tropical plants with the colorful leaves and the even more colorful flowers on top are kind of a crap shoot in the D.C. region. If you live in the heat sink of the city, leave them in the ground. If you live in one of the outskirts, where it gets really cold, take them out of the ground immediately after the first frost and store them for planting next year as above. Or, cut them down at the soil line after that first light frost, and cover the roots with a 3-inch mulch of shredded leaves.
  • And, if you live in an in-between area? “I got just one question for you: Are you feeling lucky, punk? Well, are ya?”

Mike McGrath was editor-in-chief of ORGANIC GARDENING magazine from 1990 through 1997. He has been the host of the nationally syndicated public radio show “You Bet Your Garden” since 1998 and WTOP Garden Editor since 1999. Send him your garden or pest control questions at MikeMcG@PTD.net.

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