Houseplant Winter Care Q&A

In a recent episode of Granite State Gardening, horticulturist Emma Erler joined host Nate Bernitz for a live talk about winter care of houseplants. Topics discussed included moving plants from outside to inside, dormancy, managing challenging indoor conditions over winter, and much more. That talk can be viewed below.

We didn't get to all the questions viewers had for us, so we are sharing them here as a written Q&A. Enjoy!

Questions and Answers with Emma Erler

Lisa asks: A friend gave me a rosemary plant from his herb garden. How do I keep it happy through the winter -- it seems to dry out quickly.

Emma: Keeping rosemary alive through the winter is a real challenge. One of the main keys to success is starting with a plant that was grown in a container all season long. Plants that are dug out of the garden often don't survive due to the stress of repotting. Rosemary plants also need to be placed in the proper location inside the home. They need bright sunlight and should ideally be set in front of a south facing window. We have also found that growing rosemary in a cooler room, somewhere between 45 to 55 degrees F, can improve survival. Watering rosemary is where things get the most difficult. It is important to wait to water until the potting soil is very dry, water well enough that water drains from the bottom of the pot into the saucer. You can expect to have to water less when plants are kept at cooler temperatures. 

Rob asks: Leaves curling on rosemary - beginnings of mites, but treated with neem oil. Moisture problem maybe?

Emma: Curling leaves is often a sign of moisture stress. Watering is the single most difficult aspect of keeping rosemary alive through the winter. Plants often dry out quickly and can suffer from drought, but they will also rot if they are kept too moist. We recommend keeping the plant in a room that stays between 45-55 degrees F, to reduce watering needs. 

Another option for dealing with spider mites is to spray the plant foliage with a heavy stream of water every day or so. This will knock the small mites off of the leaves, and they are often unable to successfully make it back to the plant. 

Terri Anne asks: I have 3 poinsettias from last Christmas, I couldn’t throw them because they are still healthy. With that said, they are green. Will they turn red again?

Emma: Your poinsettias have the potential to turn red again if you give them the right treatment. Poinsettias are "short day" plants, which means that they must experience 8 to 10 weeks of days with less than 12 hours of light. If the plants are exposed to any light, even artificial indoor lights during the night, flowering will be delayed. You can initiate short days by keeping poinsettias in a completely dark space from 5 p.m. to 8 a.m., and moving them into a sunny location from 8 a.m. to 5 p.m. every day. Once color is visible it is no longer necessary to keep them in complete darkness at night. Because it takes so much effort to get poinsettias to rebloom, many people find it much easier to start over with a new plant each year. https://extension.unh.edu/resource/poinsettias-care-and-reflowering-fact-sheet 

Lisa asks: Blossoms have fallen off my orchids, is there any hope of them reblooming, if I keep patiently nurturing them?

Emma: Healthy orchids should rebloom about a year from when they started to flower this year. A trick to getting them to bloom is to give them cooler nights. Even if yours are near a window they may not be experiencing enough of a chill to start forming flower buds. A good option is to turn down the thermostat by at least 10 degrees at night. You should also research how much light exposure the orchid species you are growing need. Moth orchids (Phalaenopis), need intermediate light, so an east or west facing window will do. Fertilizing and repotting in the spring with bark-based potting mix can also help.

Bernadette asks: Please tell me how to keep enough humidity around a very big rex begonia. No room for a pebble tray.

Emma: Your best option would be to increase the humidity using a humidifier. It is also helpful to decrease the temperature in the room where you keep the plant, as this will keep the relative humidity a bit higher. Making groupings of plants around the begonia may also be helpful for increasing the humidity in its immediate vicinity.

Tina asks: What can prevent fungus gnats? I seem to get them every year!

Emma: The best thing you can do is let the houseplant growing media go dry between watering, particularly the top couple of inches. Dry soil decreases the survival of fungus gnat larvae and eggs. You may also find it necessary to re-pot houseplants every few years, because the growing media will break down over time and begin to retain too much moisture. Also, make sure your plants aren't in pots that are too big for them. Ideally, plant roots should reach the edges of the container without being pot-bound. Another option is to place yellow sticky cards on the edge of containers. The adults are attracted to yellow and will get stuck to the cards. This may help you trap enough egg-laying females to impact the population. 

Heather asks: I put a bunch of stones on the bottom of my decorative pot to help with drainage. Is that okay?

Emma: As long as your pot has a drainage hole in the bottom it is fine to place stones in it. However, stones used in lieu of external drainage can be problematic. Water the pools among stones will leach into the potting mix at the bottom of the pot, and keep plant roots much wetter than they should be, encouraging root rot. It is in your plant's best interest to re-pot into a container that has at least one hole in the bottom that allows excess water to escape. 

Sue asks: I spray mine with oil spray to prevent mealy bugs...anything else I can do as preventive?

Emma: Rather than try to prevent mealybugs, it is more important to monitor for them regularly. We don't expect that neem oil would have much of an impact on meal bugs, even if they are present. Instead, for a small number of plants you can achieve good control by using a cotton swab dipped in rubbing alcohol to remove them, or washing plants with a mild dish detergent solution with a gentle brush or cloth. If you've never had issues with mealybugs before, there is no reason to expect an infestation unless you've recently brought a new plant into your home. 

Isaac asks: What should I be aiming for a humidity level for my tropical plants. Should I separate my aloes?

Emma: We recommend aiming for around 40 to 50 percent humidity. Many tropical plants will enjoy even higher humidity than that, but it is difficult to create those conditions outside of a terrarium or heated greenhouse. Aloes are very forgiving when it comes to humidity. They will grow quite well in both dry and humid environments. If space is at a premium near a humidifier, placing the aloes in a different location is perfectly acceptable. 

Sarah asks: If you do the soak method for watering, do you check the bottom for moisture to know when to do it again?

Emma: You should only need to soak pots if the the potting mix was allowed to dry out completely. This can be a messy, more labor-intensive process, so we recommend only doing it on an as-needed basis. You'll know it's time to water again when the top few inches of soil are dry or the pot feels light. 

Jillian asks: If they must be kept near a heat register, can a humidifier negate that?

Emma: A humidifier could certainly help, though it probably won't be able to completely compensate for the hot, dry air coming out of the heat register. For this reason we recommend keeping sun and heat-loving succulents closer to heat sources, and tropical foliage plants further away. If you start to notice the tips and edges of the leaves of your plants turning brown, that is an indication that the humidity is too low for their liking. 

Isaac asks: My bird of paradise’s leaves are super curled. How do I fix?

Emma: Curling leaves could be a sign of moisture stress. Plant leaves will often roll inward in an effort to reduce moisture loss. Potting mix should be allowed to dry out in between watering, but it's important to water again before plants start to show signs of wilting. Exposure to cold temperatures could also cause a Bird of Paradise's leaves to curl, as could root-rot from over-watering. 

There is no way to fix curled leaves, but if you keep the plant healthy with proper watering, light exposure, and fertilization during the spring and summer, the new leaves should be symptom-free. You should also expect to have to re-pot this plant every few years as its roots fill up the container. 

Tulsi asks: Any advice on Vincas or trailing plants? Are they okay to place in low light areas above tall shelves?

Emma: Vinca isn't usually grown as a houseplant. Vinca minor is a shade-loving hardy perennial, and Vinca major is a annual that is commonly grown in containers. It might be possible to grow Vinca major indoors, but it should be kept in a location with bright, direct light. For low-light areas we recommend pothos and heart-leaf philodendron. 

Vinny asks: How about hard water? I have my rain catcher that has a bunch of rain water left, but when that runs out should I worry about our homes hard water or should I boil water and use that?

Emma: It is perfectly acceptable to use hard tapwater for houseplants. Hard water actually contains large amounts of calcium and magnesium which are important for plant growth. 

Heather asks: Is there a better time of day to water indoor plants in the winter? Ex : morning, mid afternoon.... ?

Emma: We always recommend watering in the morning when temperatures are higher and foliage will dry out quickly. However, if you can manage to water your plants without wetting the leaves you can get away with watering at any time of the day. 

Marlo asks: Do you have any suggestions for making sure plants stay warm when they’re housed on windowsills in the winter?

Emma: If your windows are very drafty you might find it helpful to cover them with plastic during the winter months. Clear plastic will block some light from entering your home, but not enough to seriously impact the growth of most houseplants. Your heating bill will also be less. If your windows are fairly new, simply keeping your plants a few inches away from the glass is usually enough. 

Merry asks: Hibiscus, lemon or avocado plants - Artificial lights?

Emma: All of these plants get too big for artificial lights to be practical in most homes. They should do fine near a bright window in the winter months, and will really thrive if you can move them outdoors for the summer. 

Priscilla asks: I have two palms. Is there anything special to do for tropical plants?

Emma: The palms that are commonly grown as houseplants don't require any special treatment in the winter months. They should continue to grow well if they are given bright, indirect light and are watered whenever the top inch of potting media is dry.

Linda asks: Why do my African violets look like they rot out after they bloom?

Emma: If the bases of the African violets are rotten then the plants have probably been getting too much water. Like most other houseplants, African violets are most successful when the soil is allowed to dry out in between watering. It is also critical to avoid getting the leaves wet. Cold water can damage African violet foliage and cause it to decay. It is important to only apply water to the soil, not the leaves or flowers. 

Lynn asks: Can you spray lavender or peppermint oil around the pot to keep insects away?

Emma: Lavender or peppermint oil are unlikely to deter most insect pests. They are sometimes recommended for keeping rodents away, but their efficacy is limited. 

Courtney asks: does temp of water matter? Should it sit before waterng plants?

Emma: Room temperature or slightly warm water is best for most houseplants. Very cold water has the potential to damage some tropical plants. If you have a well you can use the water directly out of the tap. However, if you use city water it's possible that chlorine and fluorine have been added to it. Some, plants are very susceptible to damage from these elements, such as Dracaena, Cordyline, Maranta and Calathea. You can alleviate issues by letting the water stand for a day or more, so that some of the chlorine and fluorine will be released from it.

Dee asks: Do house plants do better when their roots tight to the pot?

Emma: That depends on the type of houseplant that you are growing. Some plants grow best when their roots take up much of the space in the pot, and don't need to be transplanted for many years, such as bird of paradise. However, plants that have high moisture requirements should usually be repotted when they become potbound, or else they will need to be watered constantly and will suffer whenever the soil dries out. Plants that prefer higher moisture include begonias and ferns. 

Mary asks: I receive a large Corn plant. What’s the best care for it?

Emma: Corn plant (Dracaena) prefer medium light, so they should be grown near an east or west facing window, or a few feet back from a south facing window. They are also sensitive to over-watering, so only give the plant water when the upper inch of soil is dry. Additionally, corn plant foliage can be damaged if you water them with tap water that contains chlorine or fluorine. The best way to alleviate this issue to let tap water sit for at least 24 hours before watering so that some of those elements will evaporate. 

Vicky asks: When should you change your Christmas catus soil?

Emma: Ideally, Christmas cactus should be repotted every three to four years. Overtime potting soil breaks down and can hold onto too much moisture or block drainage holes. Even if your plant doesn't need to be moved into a larger pot, adding fresh potting soil should improve plant health. 

Leah asks: If we use grow lights, does that interfere with the winter dormancy period?

Emma: Using grow lights for tropical plants isn't a problem at all. Most of these plants don't have a dormancy period, as they live close to the equator in the wild and experience long days year-round. However, putting temperate perennials under a grow light might confuse the plants and keep them from going dormant when they usually should. 

Amber asks: What about leaves falling off croton?

Emma: Croton often lose leaves from either under or over-watering. Make sure you are only watering when the top inch of soil is dry, and give the plant enough water so that some drains out from the holes in the bottom of the pot. Water that collects in the saucer beneath the plant should be emptied within an hour. 

Estella asks: Is it true that you can get ride of the web like and mites with a cotton qtip with alcohol?

Emma: That method might work, but it would be incredibly time consuming. Also the alcohol might damage the foliage of certain houseplants. You might find it easier to spray your plants with a heavy jet of water every few days to knock the mites off of the foliage, and/or treat with horticultural oil or insecticidal soap, following the instructions on the product label. 

Jo asks: Trying to keep parsley and basil over the winter - any tips?

Emma: Keeping basil and parley indoors over the winter won't be an easy task. Basil is a true annual plant, so it will likely be difficult to keep the same plant alive throughout the entire winter. You may need to seed a new crop once or twice if you expect to have a continuous harvest. Parsley on the other hand is a biennial crop. It typically goes dormant in the winter, and then starts growing in the spring, forming blossoms in it's second year. You may also need to seed this crop a few times to in order to meet your needs.

One of the keys to keeping herbs alive is making sure they get enough light. Basil and parsley need a minimum of six hours of direct sunlight a day. An unobstructed south facing window may provide enough light, but if you don’t have one of those, consider purchasing a full spectrum grow light and placing your plants beneath it for 14 to 16 hours a day. You’ll know if your herbs aren’t getting enough light if their stems become thin and spindly. 

Basil and parsley also need warm temperatures to grow. Keep them in rooms that are at least 65-75°F during the day, and no less than 55-60°F at night. Even though many herbs can survive at lower temperatures, they are unlikely to produce much growth under those conditions. Basil will fail entirely if it is exposed to temperatures lower than 50°F for an extended period of time.

You'll also need to keep the humidity up around these plants to keep them helpy. A humid environment can be created with a humidifier or by filling a pan with moist pebbles and placing the herb container on top, making sure that the bottom of the pot is not submerged in water. 

Sarah asks: what books do you recommend?

Emma: The Complete Houseplant Survival Manual (2005) by Barbara Pleasant, The Indestructible Houseplant by Tovah Martin (2015), and The Houseplant Encyclopedia by Ingrid Jantra (2001) are a few of my favorites.

Jillian asks: What about fertilizer for indoor herbs in the winter?

Emma: We recommend fertilizing herbs with a half-strength, complete liquid soluble fertilizer once every week or two, depending on growth. Complete fertilizers contain all three macronutrients, nitrogen, phosphorus and potassium. 

Jennifer asks: How to keep geraniums over the winter?

Emma: Geraniums usually do quite well indoors as long as you have sunny window to place them in. They will also tolerate cooler temperatures, and can be grown in any room that stays above freezing with a bright window. They won't grow very much at cooler temperatures, but will pick up again quickly in the spring with longer days, warmer temperatures and a dose of fertilizer.

Heather asks: Does spraying them with water help with humidity?

Emma: Spraying houseplants with water can definitely increase humidity around them, but only temporarily. In order to keep the humidity up, you'd likely to spray them at least a couple of times an hour throughout the entire day, everyday. You can also run into issues with fungal diseases if the foliage gets too wet. What you really want is to increase the amount of water in the air, not on the plant itself. Pebble trays and humidifiers are usually much more practical solutions. 

Barbara asks: How do you deal with mealybugs?

Emma: Mealybugs can be a challenge to eliminate because they are very good at hiding on plants. If you have a small infestation on just a couple of plants, the easiest approach is to remove them with a cotton swab and a mild dish detergent solution. This process often needs to be repeated several times. There are also pesticides that are labeled for mealybug control, such as horticultural oil, insecticidal soap and neem oil. If you use one of these products, make sure to read and follow all of the instructions on the product label. 

Andrea asks: How do I prevent mold growth on soil surface? This only became a problem in last few weeks.

Emma: Fungi and slime molds will sometimes grow on the surface of potting media, but they don't harm plants or their roots. These organisms feed on dead organic matter, primarily the components of the potting mix itself. They are more likely to appear when the potting mix stays wet for long periods of time. Simply allowing the potting mix to dry thoroughly will often make fungi and slime molds disappear. 

Archana asks: Can basil last year around?

It is possible to grow basil indoors, though it may require artificial lighting, careful watering and a humidifier. Learn more: https://extension.unh.edu/blog/how-can-i-keep-potted-herbs-supermarket-alive-longer 

Isaac asks: Should I use an air purifier?

Emma: There shouldn't be any need to use an air purifier around your houseplants. In fact, some plants will actually help purify the air themselves. 

Calise asks: how do you get rid of fruit flies?

Emma: Fruit flies are attracted to sweet liquid from decomposing fruits, vegetables, soda, or juice. Eliminating the things that are attracting them is the first step. Ripe or damaged fruit should be kept in the refrigerator or thrown away and spills should be cleaned up. Once you've done these things you can make a fruit fly trap by filling a small glass partway with apple cidar vinegar and a drop of dish washing detergent, and covering it with plastic wrap. Poke a dozen or so holes in the plastic with a toothpick and place the glass near where the fruit flies were active. The flies will crawl in through the holes and then drown in the vinegar. 

Sara asks: Have you ever propagated a zz plant?

Emma: Yes, ZZ plant can be propagated from cuttings fairly easily. You can either start plants with a section of stem and at least two attached leaves, or by single leaves. We've had success by sticking leaves into sterile peat-based potting mix, stem end down. If you're looking for more specific instructions, check out this great article from researchers at Michigan State University. 

Sonny asks: I am new to having houseplants and I have an issue with the tips of some of my plants browning and not looking good. They have only been indoor plants.

Emma: It looks like you're growing spider plants (Chlorophytum) and a corn plant (Dracaena). The tips of the leaves of both of these plants can be damaged if you water them with tap water that contains chlorine or fluorine. The best way to alleviate this issue to let tap water sit for at least 24 hours before watering so that some of those elements will evaporate. Another explanation is that your plants haven't been watered enough. Leaf tips will often die if plants are allowed to get too dry for too long.