How to Care for a Winter “Greenhouse” in Your Kitchen

As activity outside on the balcony slows down, I’m keeping busy inside this December. The shelves installed in my kitchen window (a.k.a. “the greenhouse”) are populated with a collection of scraggly young plants that will be spending winter indoors. 

Each little pot contains a miniature experiment in propagation. Cuttings of herbs, salvia, and Agastache mingle with a litter of Clarkia seedlings. From my beloved California fuchsia plant, I’m growing both cuttings and seedlings for good measure.

Many gardeners practice fall sowing and planting, mimicking nature’s rhythms. Although the plants grow slowly above ground in the darker months, they spend this time developing strong roots. By March, my kitchen window babies should be ready to burst forth and fill my early spring garden for almost no money.

Equally valuable, having something to tend to everyday helps fight off winter blues. If you’d like to get a head start on your spring garden or you too have a little winter greenhouse going, here are tips on over-wintering young plants.

Sow seeds in the fall

I’m quite envious of gardeners who grow in the ground and can simply scatter seeds outside in the fall. Many plants drop their seeds in autumn, so they’re just giving nature a hand. The seeds germinate with the right combination of rain and temperature and the plants sink their roots deep into the soil, growing away happily.

In the limited environment of a container, the seeds I’ve tried to direct sow outside in the fall have suffered. And this year, rats ate all my Claytonia and California poppy seedlings before they even had a real shot. I was hoping these cool season annuals would beautify the balcony throughout the winter.

Though I had to give up on that dream, there is another reason to sow annuals in the fall: bigger, stronger plants come spring. As opposed to starting seeds in February, when the plants might grow faster with the lengthening days but not be ready to plant out until April or May, fall-sown annuals will be ready weeks earlier with sturdier root systems. 

Fall sowing tips

For this scenario, the idea is to sow the seeds in September or October when it’s still warm and bright and then protect them through the winter. I’m following this approach for my Clarkias this year, but you could try it with cosmos, sweet peas, and other tender spring annuals.

Here are some suggestions for successful fall sowing:

• Start seeds outdoors when the weather is still decent and days aren’t too short. See this post for general seed-starting advice.
• If your seedlings grow big enough before winter hits, pot them up individually into regular potting mix to ensure good root development.
• Transition them indoors gradually. Start by bringing seedlings in at nighttime when overnight temperatures drop to the low 50’s so they can adjust to the indoor environment. Although it never freezes in Oakland, once we start getting gloomy, blustery days, I bring them in full time.
• Inside, find a spot with plenty of light, such as a south-facing window. Read on to the next section for information on watering and feeding.
• Once the days get shorter, their growth will slow. Don’t worry; they’ll perk up as the spring equinox approaches.

Indoor care tips for seedlings & cuttings

Keeping young, vulnerable plants healthy inside through the dark months requires a fair amount of fussing, but for me that’s the fun part! I’m following the general rules below for both my cuttings and seedlings:

• Keep an eye on them. Take advantage of the fact that they’re right there in your house and inspect the plants frequently. They’re not used to the indoor environment, which tends to be drier and more stagnant than the outdoors. They may have new needs from day to day.
• Water whenever the soil dries out, which will depend on how often you turn on the heat and the strength of the sunlight through the window. I sometimes spritz mine with a spray bottle to keep the humidity up between waterings.
• During holiday travel, place the pots in shallow baking trays filled with water so they don’t dry out.
• They need air circulation to avoid diseases, so keep windows cracked open as often as you can bear to. Remove any dying or potentially diseased foliage that seems suspicious.
• Stop feeding plants once growth slows, probably around late November. You don’t want to stress the plants or cause them to outgrow their pots before spring. Slow and steady wins the race!
• Start feeding with a liquid fertilizer when you start to see new growth in February and March.

Hardening off in the spring

In an ideal world, this scrappy winter greenhouse crew will be healthy and lush once March rolls around. They’ll be outgrowing their shelves, raring to break free from the confines of the kitchen and live their best lives in the fresh air. They will have to be patient though, because I’m going to harden them off.

Hardening off is the process of slowly exposing indoor plants to the outdoor elements to avoid transplant shock. Start by leaving them outside in a protected area during the day, where they won’t experience a lot of wind or harsh sunlight. After a few days of that, let them get a little sun and see what happens (unless they are shade-loving plants). 

After a week or two of daytime outings, you can leave them out overnight if the weather is cooperating. Exact temperature depends on the hardiness of the plant, but most will tolerate mid-50s and up. Spring tends to be an erratic season, so keep an eye out for cold snaps. Wait to pot them up until they are thriving outdoors full-time.

Overwintering: Nothing to lose, everything to gain

Two or three months from now, we’ll find out how many of my propagation experiments actually succeeded. Thankfully, the stakes are low; these plants cost me next to nothing, so any success will be a gain. 

Mentally, I have already gained a lot just by bringing a bit of my garden indoors. All of the cuttings were taken from my own plants, some of which were grown from cuttings themselves. I love the thought of regenerating certain plants, and the memories I associate with them, year after year.

And with the sun setting at 4 p.m. and foggy mornings outnumbering sunny ones, I love starting my day inspecting the tender leaves of new seedlings. It’s such an unlikely sight in December, but it’s one I don’t want to live without. From now on, I think I’ll always sow seeds in the fall.