Cacti are hardy plants that can grow in the harshest of conditions – from scorching heat to sub-zero temperatures. They do not require much from their soil, other than to serve as an anchor and to keep excess water away from their roots.
They are popular house plants as they are perceived to be minimalist and low maintenance.
When a cactus is grown outside of its natural habitat, care needs to be taken to emulate its preferred growing environment.
Now, if you are not successful in mimicking a cactus’s natural environment entirely, you might face some problems with your prickly friend.
One of these issues is browning.
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What could cause a cactus to turn brown?
Most of the reasons for discolouration in a cactus are caused by its environment. The temperature or light levels may be too high or too low, or the plant may be absorbing too much water. If the environmental factors have been ruled out, the problem may be due to an infestation of pests or fungus. Finally, the plant may be turning brown due to the natural aging process.
Environmental factors that cause browning – outdoors
If your cactus is turning brown from the tips, this may be due to harsh light levels. The archetypal cactus looks like it belongs in the blazing sun but this is not true of all species. Some cactus plants prefer dappled sunlight or shade.
Sunburn may suddenly occur on a cactus that has been doing well in the sun, if there has been an abrupt change in temperature, from icy cold to searing heat.
If the browning occurs at the base of the plant and the stem has a spongy feel to it, this indicates that the plant is absorbing too much water from the soil.
The plant will not enjoy a period of prolonged cold and wet weather. Cacti like to be dry in winter.
Soil that does not drain well will cause water to linger around the roots of the plant longer than it prefers. The roots will rot and the base of the plant will be damaged, appearing brown.
Cacti stems are designed to expand to absorb available water. Frost will damage the cells of the cactus if it has absorbed too much water in the cold season.
If the temperature drops below freezing, the water in the cells will freeze and expand further, causing the cells, and possibly the stem, to split.
Eventually, all cell damage will be evident on the outside of the stems.
Depending on the size and severity of hailstones, a hailstorm could damage the soft stems of the cactus and cause them to turn brown in time.
Environmental factors that cause browning – indoors
Most problems with cacti turning brown indoors will be caused by overwatering, watering out of season, or excessive humidity.
Cacti grow in response to increased light levels and will require more watering during these growth spurts. However, they like the soil to dry out significantly before being watered again.
Prolonged exposure to an air vent or heating duct may cause irritation to the plant and subsequent cell damage and browning.
In an enclosed space, that is not adequately ventilated, moisture will build up and cause problems for a plant that thrives in a dry climate.
A sudden change in any of the environmental factors – heat, light, soil and humidity – could cause the plant to lose condition and start browning.
As with cacti planted outdoors, poor drainage will cause moisture to build up around the roots of the plant, causing them to rot and will damage plant tissue at the base of the plant.
Pests, parasites and fungi
Fungi and bacteria thrive in an environment where there is decaying organic matter. If the plant develops root rot and associated dying off at the base, these organisms will seize the opportunity to colonise your plant, causing further damage.
Mealybugs are partial to cactus plants. They are small, greyish bugs that look like mildew. They breed rapidly, and will not take long to collectively cause significant harm by sucking out the sap of the plant. Dead cells that result from this assault, create opportunities for fungi and bacteria to feed.
Scale is a small bug that derives its name from its appearance. It has a tough exterior and, like the mealybug, excretes a sticky substance known as honeydew. This attracts ants that provide protection for the bugs. If ants are attracted to your cactus, look for these pests as the source.
Spider mites are tiny, reddish brown creatures that congregate on the underside of the joints in a cactus. Their tell-tale signs are white web-like structures, and their damage takes the form of small brown spots.
Aging
Most cactus plants that are removed from their natural environment are fairly young plants. It takes a while for a cactus to mature and form ‘bark’. This process is known as ‘corking’.
The brown areas that appear on your cactus as a result of this maturation, will be hard, unlike the soggy browning that arises from too much moisture in the plant.
Solutions and guidelines
Once a cactus has started to turn brown, the chances are good that the internal damage is severe. It is best to put preventative measures in place to avoid the loss of a plant.
Below are a few guidelines for ensuring that your cactus has a long, happy and healthy lifespan.
Study the environmental requirements of your particular cactus species before finding a home for it. Pay attention to the light levels, temperature, humidity and soil. Once you have positioned the plant, regularly monitor its condition. Respond timeously to any signs of stress.
If your plant is kept outdoors, minimise its exposure to extreme weather conditions such as harsh sunlight, frost and hail damage.
Try not to expose your plant to shocks in its environment. If you are bringing a new plant home from the nursery or moving it from indoors to outdoors, or vice versa, help it to acclimatize gradually.
Incrementally expose it to its new living conditions.
Strange as it may seem, your cactus plant will not appreciate the freedom to expand into a much larger pot. When repotting the plant, choose a pot that is marginally larger than the previous one.
If the aim is to ultimately plant your cactus in a large pot, do so in stages.
Cacti have evolved to live in dry conditions. They do not tolerate wet roots or soggy stems.
They have definite seasonal watering needs. In summer, when light levels increase they will have a growth spurt and will require additional watering.
Water sparingly in winter and build up gradually to the summer requirements. Taper the frequency of watering off again in autumn. Always water deeply but allow most of the soil to dry out before watering again.
Ensure that the soil drains well and that water does not clog the roots. Part of the roots’ function is to draw air into the plant. If the soil surrounding the roots is waterlogged the plant may send out aerial roots as a signal that it needs air.
Another reason that a cactus sends out aerial roots, is the presence of excess moisture in the atmosphere.
Cacti prefer dry environments but they are opportunists and will absorb water where they can find it.
Ensure that the plant is in a well-ventilated area to prevent the build-up of mold and other fungi.
As discussed earlier, the plant will suffer from being in a draft, regardless of the temperature.
Keep it away from air conditioning and heating vents.
Inspect the plant regularly for pests and parasites. If you do find any, isolate the plant from others in your home or garden. Spray it down with water under pressure, ensuring that the water drains well.
Apply rubbing alcohol to the leaves to deter their reappearance.
If your plant is indeed turning brown because of root rot, let the plant dry out thoroughly.
Cut away the damaged areas, ensuring that the plant can still support itself afterwards. Clean the roots, change the soil, and repot the plant in a slightly larger pot.
Daniel has been a plant enthusiast for over 20 years. He owns hundreds of houseplants and prepares for the chili growing seasons yearly with great anticipation. His favorite plants are plant species in the Araceae family, such as Monstera, Philodendron, and Anthurium. He also loves gardening and is growing hot peppers, tomatoes, and many more vegetables.