Should you fertilise indoor plants in winter?
The short answer is yes, but the reasons might surprise you. Especially about what roots get up to below the surface during winter. Oh, and you also should not fertilise the same way in winter as you normally do in warmer months. Read on for 'the tea' on the (sometimes controversial), question of whether you should fertilise your indoor plants in winter...
#1 Indoor plants don't go dormant
The first thing to keep in mind is that most of our indoor plants do not go dormant over winter. Most come from habitats that remain warm and humid all year round, with more consistent temperatures and sunlight levels than we offer them, so are used to growing all year round.
In fact that's a key reason they make great indoor plants after all. Partly because many grow in low light in jungles and forests, so our 'bright indirect light' indoors is a good match for the low light conditions they grow in outdoors... and partly because we heat our homes in winter.
Many of our favourites, including Pothos and Alocasia, are native to Southeast Asia, where temperatures average around 27 degrees all year round. Many Philodendron varieties are native to Tropical America, a region that includes the Amazon rainforest in Brazil, where the average temperature remains around 25 degrees all year.
#2 Indoor plants don't need a rest
Some argue plants 'need a rest' in winter and shouldn't be 'forced' to grow all year-round. That certainly is true for plants that die back in winter (like Caladium). However most of our tropical indoor plants come from habitats that stay warm all year round, so don't feel mean! You're not forcing them to grow. Rainforests in the tropics have a 12 month growing season.
What we do need to worry about providing them with in winter, is the warmth, nutrients and humidity they love, so they can grow all year round like they want to. Growth will still slow down, with cooler temperatures and reduced daylight hours. Even the Amazon has periods of faster and slower growth.
#3 Roots don't go dormant
Even when plants appear to go dormant, and a plant stops growing above-ground, there may still be activity going on under-ground that we can't see. Roots have their own pattern of growth, which doesn't necessarily follow the same rate of growth as the leaves.
In general, roots tend to show growth spurts in early summer and late autumn to bulk up before winter. But what happens to roots during winter? Different species and regions do have different patterns, however if the soil itself stays above freezing, even outdoor plants that appear dormant above ground, will divert their energy to growing below-ground through winter.
Over winter, plants go into storage mode. Think of them like recharging their batteries. After dealing with high water loss and fuelling fast foliage growth in the warmer months, plants work to both recover and to build up their stores again over winter, ready to power the next rapid growth season when it warms back up and daylight hours increase again.
If starved of what they need over winter, plants can't fully recharge their battery below-ground. It may not make any visible difference above-ground over winter, but come spring and summer, means you may not see the same foliage growth and flowering you could have. Our plants definitely 'play the long game'. The reward for looking after their needs over winter will be seen next growing season.
#4 Nutrient storage increases over winter
As part of 'recharging' their batteries in winter, plants also increase their nutrient storage, conserving energy ready for that first explosive growth next growing season. Think of it like filling up the fuel tank in winter, ready for putting your foot on the accelerator in summer.
If there's a shortage or deficiency of nutrients in winter, it's not until summer that you notice the effects, which include slow or delayed growth, stunted growth or even loss of leaves, when it should be full steam ahead. It's easy to blame more immediate actions for growth issues in summer, but in fact it may be what we did (or didn't do), for our plants in winter, that's responsible for setting our plants up for success or failure come summer.
#5 Nutrient deficiencies in winter impact summer growth
One saving grace if you don't fertilise (in winter, or at all), is that many essential nutrients are mobile. Meaning your plant can move them around itself, such as from old leaves to new leaves. If you notice a lot of yellowing older leaves during time of growth, that can be a sign your plant is lacking something essential.
Yellowing leaves can often be a sign your plant's lacking essential nutrients from the soil. It's sacrificing older leaves by 'sucking' those nutrients out of the older growth, shifting it to keep younger leaves alive and to keep growing. If you never correct what's causing those yellowing leaves, or if you cut those leaves off, plants will keep sacrificing one leaf after the other until you fix it.
However, not all nutrients are mobile, so for some, once there's a deficiency, your plant can't correct it, and neither can you. Calcium is essential, yet immobile. Common signs of a calcium deficiency include smaller, slower, stunted or deformed new growth, or new leaves and buds that soon turn brown and die.
But by the time you see the impact of a calcium deficiency in new growth, it's too late. Reducing deficiencies (in both mobile and immobile nutrients), is another benefit of fertilising all year round (provided of course that your fertiliser is complete and balanced, and includes calcium!). If you do see signs of a deficiency from an immobile nutrient like calcium, it may be too late to fix it in that growth, but you can correct it so future growth isn't deficient. Better to avoid a deficiency in the first place by fertilising all year round.
How to fertilise in winter
Before I knew the science behind why to fertilise over winter, I did it anyway , simply because I noticed most of my own jungle did slow down, but did not fully stop growing over winter. Everyone has their own method. Mine is a mix of trial (and error), research and education. Here's what I do for my jungle...
The 'weakly weekly' method
This method is mentioned a lot online, but not always explained. It doesn't literally mean to feed every week. It means you should fertilise less, more often. So you end up fertilising lightly, every time you water.
This method helps avoid deficiencies while also avoiding fertiliser burn. 'Weak' typically means half to a quarter of what the label says. For example, if a product says to 'dilute 1 teaspoon per litre, every second time you water', you might use 1/2 a teaspoon, every time you water. I personally decide how much less to feed based on the label, so for a food that recommends monthly use, I'll usually feed about a quarter dose, or fortnightly I tend to halve.
On the topic of fertiliser burn, if you're worried about the risk, you can also look for a fertiliser without urea, or even better, also without chlorides. Two common ingredients in outdoor fertilisers (and most indoor fertilisers), that can build up in potted plants over time and cause fertiliser burn.
Since there are so few fertilisers without urea, let alone free of both urea and chlorides, another solution (no matter what you feed), is to flush water every month or so. This flushes out any excess mineral salts that have built up over time, whether from fertiliser or just the minerals in hard tap water. An occasional flush water is recommended even if you don't fertilise. To do a flush, simply top water heavily with plain water, until water pours out the drainage holes. I use a fertiliser that is free of both urea and chlorides, but I do also flush water every time I water just for good measure. Flush watering also helps replace stale air with fresh air for the roots. If you normally bottom water, a monthly top water is recommended to get rid of excess minerals.
But back to the 'weakly, weekly' method: In summer that might indeed mean watering weekly, or more often. But as growth slows and temperatures drop, so does your plant's water consumption and evaporation rate. In winter you would still feed weakly, but not weekly (the grammar nerd in me loves how this makes no sense if you read that aloud).
Depending on the fertiliser you've chosen, you might feed quarter to half-strength in winter, every time you water, or at the same dose all year round because you water less often in winter. Remember dose and frequency impact each other, so simply by watering less often in winter, your plants naturally receive less fertiliser, so you don't have to change the dose. Or you can keep watering at the same frequency if needed, but change the dilution rate.
If you were feeding 1ml fertiliser per 1 litre water in summer for example, you could change that to 1ml per 3 litres in winter, but do keep fertilising every time you water. If you use a growth booster in summer, I personally skip them over winter. If you give your plants seaweed, then like fertiliser, I'd keep up the seaweed over winter, but at a lighter dose. About once a month I usually use 2 grams of seaweed flake per 1 litre of water, and in winter I reduce that to 1 gram of seaweed per 1 litre of water.
At the end of the day, there's a lot of experimenting in how we care for our plants. I love that about our hobby. I'm always learning. This is what I've learned, and what I do as a result of that learning. But guides like this, after all, are just guides. I hope sharing what I've learned, helps you. And if it does, please feel free to share this with others too :)