Extending the Growing Season by Overwintering Tender Plants

Rosemary plant brought in for Winter (by Cyndy Crist)

Rosemary plant brought in for Winter (by Cyndy Crist)

By Cyndy Crist

I’ve just completed the annual migration of some of my favorite tender perennials and annuals from the porch and garden into the house to overwinter, since they’d never survive the Minnesota winter outside.   It’s a task that’s easy to accomplish with a little thought in advance and by following a few basic steps. Here are a few tips to share that  may help you extend your own gardening season through overwintering tender plants.

Initial Considerations

  1. Select plants that are likely to grow well in pots and in the kind of environment that most of our houses have to offer.  Although a lucky few have greenhouses or greenhouse rooms, most of us have to be able to grow plants in spaces that get limited light and tend to be more uniform in temperature and lower in humidity than outdoor environments.  Because of this, before deciding which plants to bring in, a little research about growing habits and needs will enhance your chances of success.  You can also improve your odds by being ready to provide additional lighting (Amazon and others offer a wide array of options.
  2. Although indoor plants are safe from many of the pests that can attack them outside, they aren’t immune from insects.  In fact, it seems to me that the more closed indoor environment can make insect infestations that do occur more harmful because of how rapidly they can spread.  In addition, many indoor pests (notably spider mites and scale) don’t become apparent until they’ve already done significant damage.  So, vigilance is essential, along with being prepared to combat any pests that invade your indoor garden.
  3. Overwintering Citrus Plants (Stainbrook)

    Overwintering Citrus Plants (Stainbrook)

    Be prepared to provide plants with the water they need to grow and thrive. I find watering to be the biggest adjustment when I bring in plants for two reasons. One is that my houseplants that never go outside only need to be watered once per week, while the outside-to-inside plants need more frequent watering.  The second is that I just don’t notice my inside plants as much because they are scattered among several rooms while outdoors most of them “live” on the porch.  Since watering is obviously essential, I have to work at establishing a routine for this task.

  4. Plants need more humidity than most of our houses can offer once windows are closed and the heat is on. What works well for me is a plant water and humidity tray like the one made by Carter and Holmes,  Plant Watering Humidity Tray 105 (26¼” x 6½”), which has a two-part removable grid suspended over a large base tray.  Plants sit on the grid and water is added to the base tray to create humidity as the water evaporates.  It’s easy to devise something similar by putting stones or marbles in saucers, shallow bowls, or the bottoms of cache pots (with the plants themselves in a second, smaller pot with a drainage hole) and then either watering plants until water drains out the holes or occasionally pouring water into the base. Just make sure plants never sit in water.

 Making the Move:

Begonia getting washed for indoor overwintering (by Cyndy Crist)

Begonia getting washed for indoor overwintering (by Cyndy Crist)

Once you’ve decided which plants to bring in for the winter, there are several steps for preparing them for their new home.  The plants that I keep have all lived outside in pots, so I bring them in one at a time, put them in the kitchen sink, and use the spray to wash off the leaves (top and bottom) and the pot.  I wash plant trays and cache pots thoroughly and give the plants a good, drenching watering.  Finally, I remove any yellowed leaves from the plants along with any debris from the surface of the soil and settle them into their new homes once they’re thoroughly drained and dried.  Since I bring in a couple dozen plants, this process takes a bit of time to complete but it’s easily done over the course of several days (as long as I haven’t waited too long) and worth the effort.

Prepping blueberries, lemon verbena and chili peppers for overwintering (Stainbrook)

Prepping blueberries, lemon verbena and chili peppers for overwintering (Stainbrook)

If the plants to be overwintered have been living in the ground, getting them ready to bring in takes just a little more work.  First, dig them up with enough roots to take in sufficient water and nutrients from the soil.  Second, select a pot that’s large enough to accommodate the roots and the size and heft of the plant itself without being too much larger. Cautious gardeners advocate removing as much of the garden soil as possible and repotting with sterile soil, an approach I’m certain is wise but which involves more work than I would be inclined to tackle, not to mention a deft hand with the roots. Hosing off the plant is important to remove pests, as is being sure that the pots used are clean.

Some gardeners “split the difference” by keeping plants in unglazed clay pots which are sunk into the garden during the summer and then lifted out in the fall.  This approach offers the advantage of keeping roots intact and requires the same steps that I take for the pots I move inside.  Yet another option is to plant cuttings in the late summer and early fall.  With this approach, the plants brought inside are much smaller so they have less “work” to do to keep growing and the small pots make placement in good light on narrow windowsills possible.  Some plants can also be allowed to go dormant in the winter in a heated garage or basement, but that’s a story for another day.

However you decide to transition plants inside, it’s always a good idea to start the process when nighttime temperatures fall into the 50s and before the heat is turned on inside. A gradual transition is ideal, moving plants in and out for a few days (kind of the reverse of the “hardening off” routine in the spring). Since this isn’t practical with the number of plants I bring in, I try to ease the transition by opening windows near plants for a few hours each day. Starting to bring them inside earlier than may be necessary makes this approach workable most years.

What to Grow Indoors:

Herbs brought inside for overwintering (by Cyndy Crist)

Herbs brought inside for overwintering (by Cyndy Crist)

I’ve had the best success overwintering scented geraniums, begonias, and rosemary.  They all get a bit leggy by the end of winter (although I cut enough rosemary for cooking that it stays somewhat compact), but that’s easily fixed once they’re ready to move outside.  I’ve tried other herbs, including basil, thyme, lavender, and oregano, but I’ve never managed to keep any of them alive, probably because I have underestimated their watering needs. Whatever the cause of their demise, I’ve given up on them with one exception:  I’m trying a bay tree again this year. My last three have succumbed to scale, but a favorite grower at our local market gave me his formula for treating scale (*formula included at end of post) and I’m prepared to use it this winter as needed.

Prepping Rosemary and Strawberries for Overwintering (Stainbrook)

Prepping Rosemary and Strawberries for Overwintering (Stainbrook)

I know quite a few people who have not had the success with rosemary plants that I have.  I have surmised that my luck is partly a result of the large size of my rosemary plants, which I think helps them survive the early days of my adjustment to new watering regimens in a way that smaller plants might not.  It often has some powdery mildew near the end of winter, another common problem with growing rosemary indoors, but it’s never seemed to be a real problem for my plants.  I recently read that running a small fan for an hour or so on a regular basis can combat powdery mildew by improving air circulation.  I’ll give this strategy a try this year if the needles begin to be brushed with white.

In general, I think it’s worth the effort to bring in some outside plants for the winter.  If they survive, I have the pleasure of smelling wet earth when I water them, enjoying the freshness and bit of humidity that they add to indoor air, and seeing the flowers that begin appearing in late winter as the hours of daylight lengthen and the plants begin to move into active growth.  And if they don’t make it, all I’ve lost is the little bit of time and effort it took to prepare them for an indoor home.  Why not give it a try yourself?  I’ll bet you’ll be glad you did.

** Formula for treating Scale on Bay Plants:

 1/2 tsp insecticidal soap
1/4 tsp horticultural oil
1 quart warm water
The solution can be sprayed on the plant or applied with cotton swabs or balls, depending on the size of the plant and/or the extent of infestation.

Addendum on Overwintering by Dorothy:

Overwintering Lavender (Stainbrook)

Overwintering Lavender (Stainbrook)

Here is a list of edible tender perennials that I have successfully overwintered in Minnesota.  All were placed in an unheated shop attached to our house, given minimal water, and received quite a bit of East sun through many large windows.

  • Blueberry Plants
  • Rosemary
  • Lemon Verbena
  • Strawberries
  • Bay (watch for scale)
  • Lavender
  • Citrus trees
  • Aji chile peppers
  • Scented geraniums

Humidity trays:  While I usually make my own, in the way Cyndy suggested, Amazon does have a range of trays available for purchase.  My eye was drawn to the sturdiest ones, as the pots I bring in are fairly large and heavy.   One that looked promising  to me was this Humidity/Drip Bonsai Tray – Heavy Duty Black Plastic 16.5″ x 11.0″ x 1.5″.


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2 Responses to Extending the Growing Season by Overwintering Tender Plants

  1. Carmen at #

    I brought two pepper plants inside. One is a hybrid called Carmen (my name also). Right now it has 11 peppers of different sizes, including one that turned red. All the other ten developed from flowers AFTER I brought the plant inside. I live in Little Falls, Mn (less than 9 hours of daylight right now 12/31). The two plants look fine (the second Marconi doesn’t have flowers right now but still has growing peppers). They have no light source other than the south facing wall of windows in my living room. I keep the house at 62 degrees at night and 65 during the day. I transplanted them from the ground into pots with organic potting mix. Aside from water and the nutrient in the potting mix all I do to the pepper plants is shake them a bit with my hand since peppers are self-pollinating. If you don’t do that or you don’t have a fan on, they will not set fruit inside.

    • Carmen, You definitely have a green thumb! I knew it could be done in MN, and I currently have several that I brought in that are bearing fruit. Where I usually fail is in late winter/early spring when I tend to forget about them because they are out of my sight. I’m hoping to carry them over this year, but time will tell. (I love the Carmen pepper by the way).

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