How to Overwinter Plants Indoors
This article goes over techniques for overwintering plants that must remain green and growing through the winter.
As summer fades into fall, a gardener's thoughts turn to saving favorite plants for next year. With hardy perennials and shrubs the plants can be left to fend for themselves with little more than fall clean up and in some cases a nice mulching. Annuals are going to have their last hurrah and will eventually succumb to the freezing temperatures of fall nights. This leaves us with tropicals, tender perennials and a few assorted plants that can be overwintered in the house.
Many of us have favorite plants that we would like to overwinter and keep for next spring. Which plants are good candidates to bring inside for the winter? Many tropicals and plants sold as houseplants will do fine through the winter indoors. Foliage plants tend to be better suited to overwintering indoors than full sun, flowering plants because they adapt more quickly to indoor conditions.
The following should help you decide what you can bring in and help you successfully overwinter your favorite plants!
VERY Easy Tropical
Most are very tolerant of indoor conditions and can be brought in and kept near a window through the winter.
Easy Tender Perennials
These plants tolerate indoor conditions, but usually need to be in a sunny window!
Full Sun Annuals and Perennials. These plants need the sun and warmth and are extremely difficult to keep over the winter without a greenhouse.
Supertunia®, Superbena®, Snow Princess®, Senorita® Cleome, and Superbells® as well as most sun annuals stand a better chance of surviving outdoors than indoors, though most will not – that is why we can them annuals. Unless you have a greenhouse these plants will usually only cause you grief indoors over the winter. The days are just too short without enough light indoors for them to survive.
There is nothing wrong with trying to bring any plant through the winter indoors. Your likelihood of success is higher with the plants I've listed as easy or very easy, but the worst that will happen is the plant won't make it. You can chalk those attempts up as learning experiences. Gardening is all about trying new plants or old plants in new places. Sometimes it works, sometimes it doesn't. Half the fun is in trying!
What is it about the inside of a house that makes it difficult for many plants? You probably would guess that low light levels are one potential problem. Your home has lower light levels to begin with and the short days of fall and winter also contribute to the problem. The second reason many plants don't adapt to indoor conditions is low humidity. The air inside your home, especially in winter, tends to be very dry. Many plants do not deal well with low levels of humidity. These two factors combined mean that many plants are not cut out to live indoors. Let's say that despite the many challenges you would like to try and overwinter a plant in your home.
Rules of Thumb for Overwintering Plants Indoors:
1. Choose only healthy plants
2. Bring Plants indoors before frost damages foliage
3. Treat for disease and insects before bringing plants indoors
4. Place in bright areas and add humidity using pebble trays or spray bottles
5. Be careful not to over water
6 When active growth starts in spring fertilize and prune as needed
7. Introduce your plants slowly to outdoor conditions in spring
Don’t bring sick plants inside for the winter
The first thing to do is choose the plant or plants you want to bring inside. Be sure to bring your plants inside before frost has damaged the foliage. Choose only healthy plants to bring inside as the stress of the move will likely be the final blow to struggling plants.
Getting plants ready to come indoors
If the plant is already in a pot you can skip to the next paragraph. If the plant is in the ground, use a sharp spade or shovel to dig up the plant. You will want to try and get a good chunk of the root system. Remove part of the garden soil and place the plant in a pot. Fill in with a good potting mix. Garden soil tends not to have enough air space for container plants. Keep the soil level even or very slightly above the level of the garden soil. If you bury a plant too deep it will not be happy.
If the plant has been in a rather sunny area you can help decrease the shock it will experience coming indoors by placing it in a shady spot for a week or so. This will get it used to lower light levels and make the transition easier.
To prune or not to prune
Next decide if the plant needs to be pruned before you bring it inside. Plants can generally be pruned back by as much as 1/2 without damaging the health of the plant. When pruning use a sharp pair of pruning shears or scissors, you can also use a sharp knife. Clean your utensils between each plant. Use soapy water, rub them with alcohol, or dip them in milk (odd as it may seem milk will help prevent many viruses from spreading.) Be sure to remove any damaged or diseased portions of the plant.
Clean up time
Once you have your plants potted and pruned it is time to inspect them for debris, disease, and insects. Remove any dead foliage or other debris from the top of the pot. Dead and decaying foliage is a hiding place for insects and an incubator for diseases. Clean plants tend to be healthier.
Check for any insects and treat as necessary. It is important to remove insects before plants come inside. Insect populations tend to increase and spread quickly indoors. In the case of larger insects, like beetles, you can remove them by hand. If you see aphids or spider mites you will want to use a spray to kill them. If the infestation isn't large you can probably remove them by spraying the plant with a mixture of soap and water. A few drops of dish soap in lukewarm water can be very effective means of controlling insects. Spray the plant until it is dripping with the soapy mixture, be sure to get the underside of the leaves and the stems. If you can visibly see insects (like aphids and spider mites), you may want to take a damp cloth and gently wipe off the infested leaves and stems. For obvious reasons it will be easiest to use this method outside. There are also products that you can buy to treat for insects. Safer's soap is a common one. If the plant is badly infected with aphids, mites or scale you should consider discarding the plant rather than trying to bring it indoors.
While checking for insects also look for disease. Common diseases include mildews and viruses. Mildew will generally be a white or grey powdery substance. Viruses will often cause the plant to have foliage that is yellowing, mottled, or stippled, the foliage just doesn't look right. Mildew can be treated with the same soap and water mixture used against insects. If you think your plant has a virus discard it and start new next spring.
Water the plant thoroughly before bringing it inside. Be sure to allow a good amount of water to run out of the drainage hole. This will help flush out any excess buildup of salt or fertilizer in the soil. You may want to follow this flush with a light fertilizer application.
Fill up your windowsills!
Move your plants indoors and place them in areas with bright light. If you have a sun porch you are lucky and should be able to overwinter many plants fairly easily. You can use grow lights for supplemental light if you feel the interior of your house is too dim.
How to combat the dry air indoors
To help combat low humidity place a shallow pan filled with gravel (it is best to buy gravel from the store where it will be clean, if you get gravel from your driveway be sure to wash it well) underneath your plants. Add water to the gravel. This water will evaporate keeping the area right around your plants more humid. You can also spray the area with a spray bottle a couple of times a week to help increase humidity.
Reduce or stop fertilizing
While the plants are inside do not fertilize (unless the plant is growing vigorously then fertilize lightly once a month or so). You will also need to be careful not to over water. Plants inside will not use as much water as they did outside. Water only when the top of the soil is dry to the touch. When you do water try and dampen the whole root zone (a bit of water should drain out of the bottom of the pot.) It is very easy to accidentally kill your plant with kindness.
Plants in lower light situations will have a tendency to stretch and become lighter green in appearance. Don't worry too much about this problem. If you can keep your plant alive till spring you can deal with the stretched plant later.
Going back outdoors in spring
In spring when the days start to get longer you should see your plant begin to grow more vigorously (or again, some plants may not grow through the winter). When this new growth starts fertilize lightly with a water-soluble fertilizer. Prune your plant back if it is looking stretched and unhappy. Your plant is likely to begin using more water at this time so be sure to keep an eye on how quickly it is drying out. Lightly pinching new growth will encourage your plant to branch.
Once you start getting warm days, you may want to start introducing your plant to outdoor conditions. Moving plants outside during the day and inside at night will help harden it off. Plants are like people, that first 40 degree day seems awfully cold. However, after a week of 20 degrees, 40 feels downright nice. Gradually introducing a plant to cooler temperatures (hardening off) will help it acclimate to outdoor conditions. Once the threat of frost has passed, place the plant outside permanently.