The days are shortening and the clocks have gone back – what does it mean for your houseplants? They are probably at their best right now, after the brighter light of summer. But if you have grown houseplants for a few years, you will already know how dramatically they can decline through the winter months.
It’s an obvious effect, but if you are new to houseplant growing you might not notice, with a warm house and lights one. Suddenly you see weak growth; pale leaves lacking their usual colors; dry leaf tips, or whole leaves shriveling. There are several reasons why winter is hard for houseplants, and understanding the reasons makes helping them much easier.
Light: We humans have adaptive eyes – the darker it gets, the more light we let in. So we don’t fully notice the big differences in light levels between winter and summer. The combination of shorter days, the lower angle of the sun, the distance of the sun from earth, and the relatively greater cloud cover in winter (in most areas) produce startling differences in the light that actually reaches us. One study in real-life situations found that in the morning, the average indoor light in mid-summer was 466 lux, but in mid-winter it was only 65 lux. That is a 7-fold decrease! There was an average of 46 minutes of bright light over 1,000 lux in summer but only 3 minutes in winter, just one-fifteenth. No wonder winter feels so gloomy – it really is, especially if you are a plant.
For your houseplants, already living in low light indoors, in winter they find themselves in deep gloom, making photosynthesis almost impossible.
Drier Air: Winter air is dry. Again, drier than we sense, because our focus is often on relative humidity, and if you compare winter and summer for that, there isn’t much difference. But compare absolute humidity, which is what matters more for plants, and, like light, the difference is startling. At 85 degrees (F) air can hold about 30 grams of water in every cubic meter (that’s about 1 ounce in 30 cubic feet). Drop the temperature to freezing point (32 degrees F) and it can only hold 5 grams. So the same relative humidity translates into one-sixth the amount of water. It is that air that comes into your house.
For plants, keeping leaves from drying out means having a smallish difference between the water inside the leaf and the air outside. If the gap is great, it gets harder to do, and so leaves, especially edges and tips, shrivel an die. Winter air, made even drier by most of the systems we use to heat our homes, creates a huge difference for your plants. It is like Death Valley compared to summer air – no wonder you start to see damaged foliage.
Lower Temperatures: We might heat our homes, but near windows, especially if they aren’t double-glazed, night temperatures can be very low, chilling sensitive plants right through the glass.
How to Reduce the Impact off These Difficult Winter Conditions?
You need strategies to reduce the impact of these changes on our plants, so they reach next spring in a decent condition.
If your plants can’t successfully photosynthesize, they will draw on food reserves to keep alive. Doing this all winter will deplete those reserves, so they aren’t available for the burst of spring growth, making your plants weaker and weaker, until they die.
The solution is to help them reduce their growth – push them to become dormant – so they don’t use so much stored food. How to do that?
- If you can, move you plants to a cooler place. If you check it out, you will find that different plants have different minimum temperatures they are OK with. For some it is 65, or even 70, but for most of the tougher kinds – Dracaena, Monstera, Ficus – it can be as low as 50 degrees. They will be much happier through winter in a colder place, where they can doze away the weeks.
- Move more sensitive plants to higher levels of light. Many can take a south or west-facing window in winter, but not in summer. This will give them more light – kind of like flying to Florida for the winter.
- Reduce watering. This is the single most important way of pushing your plants into that dormant state, and keeping them alive through winter. Whatever was the point when you watered – barely dry surface, one inch dry, half-pot dry – reduce it further. Dry plants will stop growing – if you see new leaves coming, reduce it even further. Plants that aren’t growing won’t use their precious food reserves just to stay alive.
- When to water in winter? Plants with thicker, leathery leaves, and cacti and succulents of course, can all live for extended periods with dry soil, so only water if leaves start to droop a little, or stems begin to become softer than normal. The water thoroughly, and don’t do it again until you see the same thing. Some more sensitive plants, like Maranta or Calathea should be watered when there is still some dampness at the bottom of the pot. If you aren’t sure, well-rooted plants can be slid out of the pot without disturbing them, so you can tough the soil at the bottom of the pot directly. You might be surprised of how damp it still is at the bottom.
- Stop fertilizing. Stop feeding your houseplants in early September. The reduced nutrient levels, combined with lower watering, will signal to your houseplants that it is time to take a rest, and will trigger their dormancy mechanisms. Once light levels rise in late winter or early spring, you can start feeding again, and return to normal watering routines.
- Avoid cold damage: place houseplants away from doors or windows that are regularly opened in winter. If you have them on windowsills, or moved them there for the winter, then make sure you close curtains between the plants and the glass, not between the plants and the room. Cold can penetrate through windows, and damage houseplants, even if the overall room temperature is OK.
What About Plants like Citrus or Azaleas?
A lot of us try to grow plants that are not winter-hardy where we live. Citrus trees – oranges, lemons, kumquats, etc. – are popular potted plants to grow on a summer terrace. Azaleas or camellias will often flower indoors if treated correctly. Yet many people find this tricky, with flowers never developing, or fruit falling. So what is the best way to overwinter them?
The important thing to realize is that these are not tropical plants. The like – and need – colder winters, just not too cold. If they don’t have that, they won’t develop properly, and simply grow leaves, without any flower buds forming, or if they do form, they won’t open. They should be kept in the brightest places possible, like a west or south-facing window for citrus, and east facing for azaleas. Plenty of light is needed to stop them dropping their leaves.
The temperatures should be as low as possible – low 50s for citrus, 40s for camellias and azaleas. Even a morning in the high 30s is unlikely to do any harm. A glass porch or sunroom is often ideal, and in all but the coldest zones having a heater with a thermostat in the space, set for around 40, is a good idea if it should get extra cold some nights. In colder zones of course you will need something that is easier to keep warm – perhaps a room where you can turn the thermostat down and keep the door closed so heat from the rest of the house doesn’t get in.