How to Overwinter Potted Perennials Outdoors
If you didn’t get all of your perennials out of their pots and into the ground before temperatures plummeted, don’t despair. You can still make sure that they that they live to see the following spring. I’m going to show you, right here, how to overwinter potted perennials outdoors.
Greetings, my dear readers. I hope these days leading up to the Holiday Season are finding you all happy and healthy. Here in Michiana Shores, along the southeastern beachfronts of Lake Michigan, these particular days are definitely on the chilly side. The wind is whipping, leaves are dropping, and snowflakes are falling. Winter’s a-comin’ here in hardiness zone 5B/6A.
At this time of year, with the holidays on my doorstep and winter right there with them, I’m typically pretty busy. There are the usual holiday activities (family get-togethers, dinners, parties), plus regular garden clean-up stuff (pruning, raking, bulb planting, etc.) which keep my plate pretty full. I’m also busy cooking up articles for this site so I can keep you up to date on my antics. And this late fall season in particular is finding me a little busier than usual. As my regular readers know, I’ve been building a pretty substantial arbor/trellis structure in my garden which has taken up quite a bit of my time. And although I’m (thankfully) almost finished building this thing, I am still working on its final component. (Believe me, when this s.o.b. is done, you’re gonna be the first to know).
So, I’ve been pretty busy. In fact, I’ve been so busy, I didn’t have time to get some of my perennials planted. I didn’t have time to get a lot of my perennials planted. They’re still sitting in their containers, instead of in the ground, where I should’ve already stuck them. (Hey, it happens). If you’re like me, and you didn’t have time to get some of your guys in the ground this year, don’t sweat it – it’s definitely not the end of the world. There are, in fact, a few things you can do to keep your potted specimens safe over the winter, and to make sure they’re healthy and ready for planting in the coming spring.
Listen, I know what I’m talking about here. I had a laundry list of different excuses for being in the exact same situation last year. Yep, I ended up overwintering a whole bunch of hydrangeas, hostas, coral bells, gaura, and various types of groundcover plants in their containers last winter because I couldn’t get them planted before the ground became unmanageably frozen. And guess what, every single one of them survived the winter and thrived beautifully this spring, summer, and fall. In fact, some of them did so well, they’re going to be spending this winter in their pots. Again. (I know, I know. Shame on me). But those plants which did overwinter in their containers last year really did do great this last growing period – the individuals I planted in the ground grew beautifully, and those which remained potted also were lush and healthy.
Many of the plants in the three photos immediately below, were planted after spending last winter in their respective pots.
So, what do you do if you don’t get the chance to plant some of your potted perennials in the earth before the snow starts flying?
How to Overwinter Potted Perennials Outdoors
In the late fall, and through the winter, most perennial plants growing in hardiness zones 6 and lower need to reach a state of dormancy for extended periods of time in order to grow, thrive, and bloom the following spring, summer, and fall. This dormant state begins when the days shorten and temperatures drop. Freezing temps generally coincide with full-on plant dormancy. But even during these periods of dormancy, a plant will continue to develop its root system and absorb moisture and nutrients from the soil. This is why fall planting can be so beneficial for so mant types of perennials. And this is why I should’ve gotten my act together and planted everything that needed planting this fall.
But all is not even close to being lost for me, for my un-planted perennials, or for others like me (and their un-planted perennials). Here’s what to do:
1. Feed them (if necessary). If it’s still late August or early September, and you’ve got perennials which you know you’re not going to be able to plant before temperatures start to plummet to freezing levels, it’s a good idea to make sure that they’ve got some nutrition at this time, but no later. If you’ve purchased them in potted form at a nursery earlier this spring or summer, there’s a good chance that they’re rooted in potting mix in their original containers. In this case, they really won’t need additional fertilizing, as the potting mix in which most growers ship their potted plants contains levels of fertilizer which will generally feed those plants for a number of months. But if you’ve grown your potted perennials from seed, or if they’re survivors who’ve remained in their pots for a second spring/summer/fall, heading towards a second winter (like a number of mine), a really light feeding is in order. I knew, back in early September, that a number of my still-potted hostas from two summers ago were not going to make it into the ground before winter. So, I added a small amount of fertilizer to their pots at this time. Because these plants are pot-bound and not growing in the ground itself, the natural process of the fertilizer leeching off into the soil of your garden is absent here. The fertilizer will remain immediately around your individual plant’s roots. So don’t use so much. Even in the best of conditions in a healthy garden, too much fertilizer is way worse than not enough. You can literally poison your plants to death by over-fertilizing them. And if those plants are in a pot, and that fertilizer doesn’t have the means of getting washed away or diluted, it’s going to stay at the immediate location of each potted plant’s roots. Too much of it will kill them. For sure. A minimal amount of fertilizer applied no later than early September will be of benefit to a plant that’s going to spend its winter in a pot. Don’t fertilize your potted plants any later than this time. You don’t want to encourage new growth, which would be damaged by the coming freezing temperatures. And even though your plant’s roots will be absorbing moisture and nutrients over the winter, it’s going to be doing this at a substantially diminshed rate.
I recommend using Espoma Plant-tone all-purpose, organic plant fertilizer. It’s got a great N-P-K ratio of 5-3-3 and it doesn’t amend the pH levels of the soil in the pots. For plants that benefit from an acidic soil, like rhododendrons, azaleas, and other evergreens, you may want to try Espoma Holly-tone. This organic fertilizer has got an N-P-K ratio of 4-3-4 and contributes to an acidic soil environment. I’ve used these great products with extreme success in my garden, and with many of my overwintering potted perennials. For more on N-P-K ratios and soil pH (acidity/alkalinity), please feel free to check out my article on lilies and daylilies here on The Renaissance Garden Guy.
RGG readers and subscribers know that I swear by Espoma organic plant foods. Click the Amazon #advertisement links directly below to learn more about, and to order, two of my very favorites.
2. Prune (if applicable). Herbaceous perennials essentially “turn off” their aerial stems and foliage when temperatures consistently drop. In this way, these plants are able to divert any and all of their energies toward protecting/developing their subterranean roots/bulbs during frigid temperatures. When you notice your lilies, hostas, peonies, etc. getting brown, shriveled, and soupy-looking, it’s best to cut them down to the ground. This is important for a couple of reasons. First, dead and soggy stems and leaves may be tempting to foraging animals which can uproot a dormant plant while grazing. Second, those dead, soggy stems and leaves can harbor various fungi, parasites, and diseases which can attack the plant at its crown and below, and ultimately kill it. These mishaps can result not only in the case of a plant which is bedded in the soil of your garden, but also in the case of a plant spending its winter in a container. Don’t take any chances. Cut your herbaceous perennials’ (potted or otherwise) soupy-looking dead stems and leaves down to the soil level for the winter.
I really like to use my Fiskars Bypass pruners for this job. They’re remarkably sharp, and they stay that way for a long time. When I use these for cutting back my deciduous perennials in winter, there’s no danger of accidentally snagging/pulling on a dead stem and posiibly damaging the plant’s crown, or even uprooting it. I’m definitely giving this brand a plug. They’re great pruners.
Note: Many herbaceous perennials can actually benefit from keeping their dead aerial structures in place over the winter. Plants like asters, solidago, and many varieties of coreopsis exhibit very dry aerial structures during their winter dormancy periods. Their dry and brittle stems and leaves make them less attractive to foragers and less susceptible to fungal attack or disease. Leaving these structures at least partially intact over the winter can help protect the respective plant’s roots during the freezing weather.
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3. Shelter your potted perennials. The best option for overwintering potted perennials is to place them in an unheated structure, like an unheated, detached garage, or a storage or potting shed. In this way, the plants will still be subjected to the cold temperatures necessary to ensure a sufficient period of dormancy, but they’ll be protected from blistering and potentially deadly winds, not to mention predation by foraging herbivores. Note: Evergreen perennials need at least some sunlight over the winter months. If you’ve got potted evergreens to overwinter, and you want to keep them inside, make sure that they’ve got access to at least some natural light for at least part of the day. A detached, unheated garage or shed with windows will probably work fine.
The next best option for overwintering potted perennials is providing them with adequate shelter outdoors. This starts with selecting a spot for them against the side of a structure (house, garage, privacy fence) with a southern or southeastern exposure. This protects the plants from icy northern winds. Although a south/southeast exposure is optimal for overwintering plants in containers, it’s not absolutely necessary. (As long as the plants are sheltered by some kind of structure on at least one side, the following steps should provide you with ample opportunity to successfully insulate your potted plants). The next step is “huddling” the pots close to one another. Once you’ve got your plants grouped together in this location, I recommend placing an elevated barrier to the exposed side(s) of your grouping. A good example of this is a layer (or, better yet, layers) of bags of mulch, soil, seed etc. laid end to end around the grouping of pots. This keeps the pots fairly well-protected on all sides. The next step involves filling in the gaps, or spaces, among/between/in front of/behind the pots with a natural insulating material such as fallen tree leaves and/or pine needles. This insulation is critical. Remember, the root systems of these plants are not protected in the ground, but are exposed in aboveground pots. This situation makes the roots of those potted plants far more vulnerable to frigid temperatures than the roots of plants growing in the ground. Those potted root systems get substantially colder over the winter months than do the roots of plants overwintering in the ground. And they freeze much faster, too. As a final insulating measure, I add a moderate layer of wood chip mulch to the surface of all of my overwintering potted perennials. This step adds an extra measure of topical insulation to each potted plant and still allows for moisture and vapor permeability. By following this methodology, you are in essence creating an insulated “bunker” for your overwintering potted plants.
4. Moisturize. Plants need water. Even in winter. Especially if they’re inside an unheated, roofed structure. Whether inside a structure or in an outdoor bunker as shown above, moisture must periodically be available to the roots of potted, overwintering perennials. If they’re outdoors, they’ll probably get most of the moisture they need naturally. If inside your unheated shed, they’ll periodically need a drink. What I did last year (with my outdoor hibernators) was pretty basic. If I knew that the outdoor temperatures would remain above 40 degrees (Fahrenheit) for a few days, I’d lightly water them. Lightly. And I wouldn’t do it constantly and I wouldn’t do it if I knew temperatures would imminently plummet. Remember, the soil in those pots freezes much more quickly (and to a lower temperature) than the soil in the ground. Soaking the roots of your potted perennials on a day when the temperatures outside are warm, only to have them freeze solid when the temperatures drop is a good way to help kill your hibernating plants really quickly. A little bit of water as temperatures trend warmer is a good rule of thumb for plants overwintering in pots inside a protected structure, and outdoors, as well.
5. Use plastic pots. Just as the temperature of the soil in an aboveground pot freezes much faster than the soil in your garden, the soil in that same pot warms up faster than the soil in the ground, too. When soil (or any substance, for the most part) changes temperature, it expands and/or contracts. Plastic is a very malleable, flexible product. This is why plumbing water supply and drain lines in many cases are made of plastic. Plastic’s flexibility makes it far less likely to crack, or rupture, when whatever it’s holding, whether soil or water, freezes and thaws, and expands and contracts. This is why it’s advisable to overwinter your un-planted perennials in plastic pots. Terra Cotta, ceramic, or glass products are far more likely to rupture or crack than is plastic. If the pot in which an overwintering perennial is planted bursts, and the roots of that plant are exposed directly to the freezing air, it will die. Plain and simple. I strongly suggest using plastic pots.
I overwintered many of my guys in the plastic pots they originally came in from the nursery or grower. Some I transplanted into sturdier plastic pots. In all cases, the plastic pots withstood the cold and any and all temperature fluctuations, and kept all my slumbering perennials safe all winter.
Video review. If you’re up for it, click the monstrous image link of yours truly (either to the right, or directly below, depending on your screen) to watch The RGG YouTube channel video, “How to Overwinter Potted Perennials Outdoors,” which will walk you through my process in real time.
If you’ve got plants still sitting in pots toward the end of August or the beginning of September, and you don’t think they’re going to make it into holes in the ground which you don’t think you’re going to dig before the snow starts flying, you might want to think about following my simple plan if you want to make a go of keeping them alive over the winter. Give them a super light fertilizing, if they need it, long before the cold sets in (no later than really early September in zones 5 or 6). If they’re the kind of herbaceous perennials which turn soggy and brown in fall/winter, prune them down to the soil level. Give ’em shelter. Let them have a drink every once in awhile (when you know it’s going to be warm for a while). Keep them in flexible plastic containers. That’s it. It’s a pretty basic system that’s really easy to implement. And none of it takes much time at all. Putting in this little bit of effort here at the onset of cold weather will go a really long way toward ensuring that those potted plants, which never made it into the ground this year, will get planted next year. Or in my case, the following year. Hopefully.
Cheers, and Happy Gardening!
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Very informative article, I always wondered how garden centers took care of their perennials after the season was over. The plants you over wintered last year did great this year. It was great to see some of your plants blooming, it’s a sign that your garden will thrive this winter and do even better next year!
Wow that’s great. I did not know I could do that!
Perfect timing for this great article. I have a question: I brought in a pot of lavender, have it in my warm plant room with grow lights. Should I move it to the dark, cool basement storage room? Thank for writing this educational and fun blog!
Hi Jill,
Thanks so much for your kind comment and for your excellent question. The answer depends partially on whether your lavender is one of the cold-hardy English varieties or if it’s a more tender variety. One thing for sure, lavender is evergreen, so regardless of its ability to stand up to frigid temperatures, I wouldn’t store it in darkness. It’s going to need exposure to at least some light over the winter. If it’s a tender variety, bring it into your well-lit plant room (or some bright spot in your house). If it’s a cold-hardy English variety, I’d let it experience the cold by prepping it as I described in this article.
My hardy English lavender love the cold weather. They’re planted in the ground in my garden and they’re definitely unconcerned by the frigid temps we experience here in the winter. Even though they do remain green through the coldest months, they definitely enter a state of dormancy where new growth stops and the plants sort of “hunker down.” Once the weather warms up, they really explode!