How to winter indoor plants? - Ask Extension
I have many indoor plants. Of these, the calla lily, purple shamrock, and mini pomegranate must be overwintered. I can’t figure out how to do so.
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Knowledgebase
How to winter indoor plants? #856445
Asked December 17, 2023, 8:27 AM EST
I have many indoor plants. Of these, the calla lily, purple shamrock, and mini pomegranate must be overwintered. I can’t figure out how to do so.
I live in an apartment that faces west. In the winter, the sun is partially blocked by another building. I have big windows, but each window has a forced air convector under it. I have no access to a cool, dark place to put my three plants that must be overwintered.
I have read many “directions” on many different sites on how to overwinter these plants. They all contradict themselves.
I am using the Planta app, which says the calla lily and purple shamrock must be let to go dormant, placed in a cool room after checking for potential pests, and for the season, reduce watering and stop fertilizing. Planta also says to do the same for the mini pomegranate, but to give it a grow light.
Is Planta’s guidance correct? If so, how do I provide these plants a cool place when there is none in my apartment? If Planta’s guidance is not correct, how should these plants be overwintered?
Thank you for your help.
Montgomery County Maryland
Expert Response
Overwintering plants requiring a "winter rest" (cool conditions when they suspect growth for a period of time) may be challenging in your situation and might not be very feasible. The ideal temperature range for Calla and similar tender (not cold-hardy enough to stay outside) bulbs is around 50-60 degrees; you definitely want to avoid temps as warm as 70. We have little information specific to keeping Calla lilies indoors for any period of time, but here is what we have shared with other gardeners based on information in our reference books:
Calla Lilies are not reliably winter-hardy here but can be (and are best when) grown outdoors in dappled afternoon shade for the summer; afterwards they are either allowed to be killed by freezes or dug up and brought indoors before the fall frosts for storage until next May when it's safe to put them back outside. Calla Lilies are often sold potted in-bloom in spring (around the same time Easter Lilies and forced potted Tulips are out) or as dormant rhizomes in spring along with other non-hardy summer-blooming bulbs. When potted, they're usually intended to only be kept as long as the flowers last. You can keep them for years longer, but it can be challenging to give them the dormancy "rest period" they need.
They do not grow or keep leaves year-round; at some point, all of the leaves will be shed and the tubers will stay dormant for a few months. N.C. State suggests storing dormant tubers dry between about 50 and 60°F. Tips from the book The Complete Houseplant Survival Manual (by Barbara Pleasant, 2005) say to "allow the plants to dry until leaves wither, clip off the old foliage, and keep the pots very lightly moist in a cool, shady spot." This is for late summer; after this autumn dormancy, potting them up again in early winter in fresh soil and increasing light, moisture, and warmth will "wake" them up and restart growth.
When growing indoors, Callas need bright light (such as in a sunny window for several hours), cool temperatures (above 60 but below 75° F, though warmer in summer is ok), and fairly regular fertilizer (perhaps monthly). Excessive amounts of nitrogen may contribute to leaf/stem flopping, as will to too little light. Plants should be consistently moist (but not soggy) when in active growth, but allowed to dry more and more between waterings in late summer until they go dormant, at which point they should be allowed to dry out nearly completely. If not allowed to grow outdoors at all over the years, they may gradually weaken and decline until they don't perform well any more. Given a spring/summer outdoors, they may live longer, though we don't have data on exactly how long.
For now, although it's a bit late, you can try stimulating this plant to go dormant by letting it dry down to greater and greater degrees between the next few waterings. Cut off all dead foliage as it appears, and when totally bare, let it stay dormant in a cool spot as mentioned above.
We don't have more detailed growing tips, but you may find some guidance from vendors of Calla Lilies (and other spring and summer bulbs) online.
As for pomegranate, it is a large-growing shrub that may not fruit well if not exposed to a full-sun location and planted in the ground to give its root system enough space to support good growth. It takes a lot of energy to produce and ripen fruits, and if a plant isn't vigorous enough by receiving enough direct sunlight to build-up food reserves in its tissues, it won't be able to flower (and thus fruit) well. We also have barely enough growing season length to allow the hardiest cultivars of pomegranate enough time to ripen fruit before freezes hit, since they flower quite late compared to other fruiting trees. When indoors, there are two options: letting the plant go dormant (usually this is done while it's still outdoors in the autumn, so the chilling air and shortening days trigger leaf drop) or keeping it well-lit and in moderate temperatures (still not too warm) while inside. If direct sunlight from a window is insufficient, you may need to use a plant grow light to supplement its light. We do not have specific information about how long of a "day" to give the plant (that is, how long to leave the grow lights on per daily cycle) since it's not in active growth while indoors.
As with any potted plant, do make sure that it is only watered when needed (so, not on a set schedule, though you can certainly check on it on a schedule just so you don't forget). For most plants, this means feeling the soil about an inch or two deep into the pot, and only watering once it begins to feel somewhat dry to the touch at that depth. Watering too often can deprive roots of oxygen to the point where they die back, which is not a condition most plants can recover from, nor is root rot treatable. Empty any saucers under a pot promptly after it collects drained water, in order to keep that extra moisture from seeping back into the soil and keeping it too wet. Using materials like stones (or the glass marbles) atop soil can hamper moisture evaporation, keeping the soil wetter for longer (and making it harder to probe for watering needs), so generally isn't advisable. By holding moisture in the surface soil, it also might make the container more hospitable for fungus gnats.
Cease any fertilizing you might have been doing to avoid overdosing the plant with nutrients it can't use while in dormancy. Dry indoor air can stress plants as well as support population booms of certain insect or mite pests, such as spider mites, so the use of a room humidifier may help avoid both issues. It will, at least, be better than misting foliage, which we don't advise since that doesn't raise humidity enough but does increase the risk of leaf infections by wetting the leaves often.
Darkness is less important in this case with regards to the "cool, dark" instructions you've encountered. The short days of winter will automatically keep many indoor plants from growing much, since they can't manufacture enough food to support it. (Despite how it's often phrased, fertilizer is not plant "food," only light, so they can photosynthesize.)
Oxalis (the shamrock) tends to go summer-dormant, not winter-dormant. Tips for care year-round are available on the University of Minnesota page "The Care and Feeding of 'Shamrock' Plants" and Iowa State's Growing Shamrocks Indoors.
If you can't provide a space in the apartment for plants needing cooler temperatures (is there a drafty window they can sit in front of?), then you might find that they don't overwinter well. Unfortunately, there isn't really a recourse if you don't have access to an area that isn't as cold as outside but which stays cooler than typical room temperature in heated indoor spaces.
Miri
Calla Lilies are not reliably winter-hardy here but can be (and are best when) grown outdoors in dappled afternoon shade for the summer; afterwards they are either allowed to be killed by freezes or dug up and brought indoors before the fall frosts for storage until next May when it's safe to put them back outside. Calla Lilies are often sold potted in-bloom in spring (around the same time Easter Lilies and forced potted Tulips are out) or as dormant rhizomes in spring along with other non-hardy summer-blooming bulbs. When potted, they're usually intended to only be kept as long as the flowers last. You can keep them for years longer, but it can be challenging to give them the dormancy "rest period" they need.
They do not grow or keep leaves year-round; at some point, all of the leaves will be shed and the tubers will stay dormant for a few months. N.C. State suggests storing dormant tubers dry between about 50 and 60°F. Tips from the book The Complete Houseplant Survival Manual (by Barbara Pleasant, 2005) say to "allow the plants to dry until leaves wither, clip off the old foliage, and keep the pots very lightly moist in a cool, shady spot." This is for late summer; after this autumn dormancy, potting them up again in early winter in fresh soil and increasing light, moisture, and warmth will "wake" them up and restart growth.
When growing indoors, Callas need bright light (such as in a sunny window for several hours), cool temperatures (above 60 but below 75° F, though warmer in summer is ok), and fairly regular fertilizer (perhaps monthly). Excessive amounts of nitrogen may contribute to leaf/stem flopping, as will to too little light. Plants should be consistently moist (but not soggy) when in active growth, but allowed to dry more and more between waterings in late summer until they go dormant, at which point they should be allowed to dry out nearly completely. If not allowed to grow outdoors at all over the years, they may gradually weaken and decline until they don't perform well any more. Given a spring/summer outdoors, they may live longer, though we don't have data on exactly how long.
For now, although it's a bit late, you can try stimulating this plant to go dormant by letting it dry down to greater and greater degrees between the next few waterings. Cut off all dead foliage as it appears, and when totally bare, let it stay dormant in a cool spot as mentioned above.
We don't have more detailed growing tips, but you may find some guidance from vendors of Calla Lilies (and other spring and summer bulbs) online.
As for pomegranate, it is a large-growing shrub that may not fruit well if not exposed to a full-sun location and planted in the ground to give its root system enough space to support good growth. It takes a lot of energy to produce and ripen fruits, and if a plant isn't vigorous enough by receiving enough direct sunlight to build-up food reserves in its tissues, it won't be able to flower (and thus fruit) well. We also have barely enough growing season length to allow the hardiest cultivars of pomegranate enough time to ripen fruit before freezes hit, since they flower quite late compared to other fruiting trees. When indoors, there are two options: letting the plant go dormant (usually this is done while it's still outdoors in the autumn, so the chilling air and shortening days trigger leaf drop) or keeping it well-lit and in moderate temperatures (still not too warm) while inside. If direct sunlight from a window is insufficient, you may need to use a plant grow light to supplement its light. We do not have specific information about how long of a "day" to give the plant (that is, how long to leave the grow lights on per daily cycle) since it's not in active growth while indoors.
As with any potted plant, do make sure that it is only watered when needed (so, not on a set schedule, though you can certainly check on it on a schedule just so you don't forget). For most plants, this means feeling the soil about an inch or two deep into the pot, and only watering once it begins to feel somewhat dry to the touch at that depth. Watering too often can deprive roots of oxygen to the point where they die back, which is not a condition most plants can recover from, nor is root rot treatable. Empty any saucers under a pot promptly after it collects drained water, in order to keep that extra moisture from seeping back into the soil and keeping it too wet. Using materials like stones (or the glass marbles) atop soil can hamper moisture evaporation, keeping the soil wetter for longer (and making it harder to probe for watering needs), so generally isn't advisable. By holding moisture in the surface soil, it also might make the container more hospitable for fungus gnats.
Cease any fertilizing you might have been doing to avoid overdosing the plant with nutrients it can't use while in dormancy. Dry indoor air can stress plants as well as support population booms of certain insect or mite pests, such as spider mites, so the use of a room humidifier may help avoid both issues. It will, at least, be better than misting foliage, which we don't advise since that doesn't raise humidity enough but does increase the risk of leaf infections by wetting the leaves often.
Darkness is less important in this case with regards to the "cool, dark" instructions you've encountered. The short days of winter will automatically keep many indoor plants from growing much, since they can't manufacture enough food to support it. (Despite how it's often phrased, fertilizer is not plant "food," only light, so they can photosynthesize.)
Oxalis (the shamrock) tends to go summer-dormant, not winter-dormant. Tips for care year-round are available on the University of Minnesota page "The Care and Feeding of 'Shamrock' Plants" and Iowa State's Growing Shamrocks Indoors.
If you can't provide a space in the apartment for plants needing cooler temperatures (is there a drafty window they can sit in front of?), then you might find that they don't overwinter well. Unfortunately, there isn't really a recourse if you don't have access to an area that isn't as cold as outside but which stays cooler than typical room temperature in heated indoor spaces.
Miri
