There is a rule of thumb with houseplants: if they are going to get stressed they often start to show it around mid-January. A couple of factors contribute to this.
First, houseplants typically head into winter with a lot of vitality after the long days of summer; by January short days have started to catch up with the plants. This effect is compounded as plants start to want to grow more, in the now obviously longer days, if there is inadequate soil nutrition for them to cycle-up. Second, by January heating systems have been running regularly for weeks on end, which dries out the air. Many plants respond negatively to the more arid conditions. Third, dust can build up on plants and most houseplants do not get regular showering/dusting; the dust reduces the efficiency of photosynthesis and weakens them. Lastly, watering houseplants can be a hit or miss event, especially after all of the distractions around the holidays (or, for the over-waterers out there, the impact of too much soil hydration may be catching up with your plants).
Although houseplant stress ranges from mild to extreme, there are four common symptoms that alert us into houseplants' need for attention: leaf drop, yellowing of the leaves, browning of the leaves, and stickiness on the leaves or around the plant. Each of these situations is associated with a different type of stress, so let's break our check-up into four categories.
Leaf drop is quite common, and typically indicates that a houseplant is responding to the lower light conditions of fall and winter - to a certain extent this is normal. However, if plants are losing leaves in January, after holding them nicely for the first few months indoors, the cause is likely to be related to under watering. Check that each watering is deep, so that soil is being fully hydrated from top through to the drainage holes at the bottom. Also, double check that the soil has not dried out to the point of having difficulty retaining moisture; if water seems to run right through, try setting the pot into a basin of water for a couple of hours, to let the soil rehydrate. Other causes of leaf drop include direct exposure to forced air heat vents as well as inadequate feeding; if everything seems okay on the watering front consider these possibilities.
Yellowing of leaves is almost always associated with over watering or lack of nutrients. If you suspect you are an over waterer, be sure that you are allowing the soil to dry to the touch between waterings. Also, double check that pots have drainage holes - this is a must for 99% of houseplants. Yellow leaves or not, if you have not been feeding your plants, start immediately; application of general purpose organic plant food or a good top dressing with a compost such as Sea Soil will allow your houseplants to respond to the increasing daylight with renewed health.
Bear in mind that some plants, such as bananas, will regularly replace older, lower leaves with new top growth - and the lower ones will turn yellow before falling off; this is completely normal.
Browning of the leaves tends to indicate dry air, particularly if the edges are the main trouble spot. Although it is difficult to avoid forced air vents in our climate, finding a way to deflect the air away from plants is helpful. Misting with fresh water also helps - as long as you remember to do it daily. Setting little dishes of water near plants improves humidity without having to remember to do something as often. For power growers (say, those with a plant room or a bunch of unusual tropicals) it may be worth setting up a small humidifier if your plants are showing lots of brown leaf edges. It is important to note that some plants, such as true tea plants or the amazing but fickle sweet olive, just get brown leaf edges each winter... not much seems to change this.
Stickiness is possibly the most dread of situations, as it is associated with pests. Pests do tend to show-up in January, as plants are often weaker for all of the reasons already discussed. If obvious pests are present, do your best to bring their numbers down swiftly. You may want to squish, or vacuum (particularly white flies), or release ladybugs (we are currently taking orders for our next shipment of indoor ladybugs; very helpful if you have quite a few plants you want to clean up). Next, shower your plants. This would ideally be part of your indoor routine, regardless of the presence of pests - but becomes extra important if pests as discovered. While plants are moved out of the way for showering, give your growing areas a little wipe down with warm soapy water, or even add a little bleach or hydrogen peroxide, to remove sticky residue and any potential pest eggs or small nymphs (some are so small you will not be able to see them). Also, remove any fallen leaf debris and consider trimming back the tops of any plants with significant amounts of pests. Lastly, you can spray plants with an effective yet safe insecticide such as Safer's End-All.
A reminder that indoor pest problems are most common when there is some other kind of stress for plants, so review the general guidelines for healthy houseplants (you can read a full article on this here) and set a date to repeat the showering in the week or two after treating for a problem.
So, set aside a few minutes to check-up on your houseplants. If things are less than perfect, don't despair - as you have read there are easy solutions to most of the common challenges, and you'll have your plants back in form in no time!
Dave Hanson has 20+ years experience helping local gardeners find success. He has co-developed Manitoba's only all-natural full service garden centre and had the opportunity to assist thousands of gardeners along the way. Dave has also been very active as a workshop presenter, speaker and media contributor for print and broadcast, including as the CBC Radio gardening columnist for the last 6 years. Dave brings passion, broad experience and a personal interest in getting to the heart of your gardening questions!