There can be a number of reasons for why your cactus is growing arms including lack of light, over-watering; keeping your cactus in an environment that’s too warm during winter, and growing your cactus in soil that is too rich in nitrogen.
When you place your cactus in front of a sunny window, it’s not the same as planting it outside in nature.
When a cactus is sitting at a window, it’s receiving sunlight from only one direction and that can contribute to elongated growth.
When a cactus isn’t receiving enough light it may develop arms in an attempt to reach for it.
When a cactus is growing outdoors, it receives light from all angles because of the way light bounces around outside and this helps the cactus to grow evenly and to its full potential.
Sitting by a window, your cactus is never going to receive as much light as it would outside in the open but there is something you can do to boost the amount of light it gets.
Create An Environment That Maximizes Light
Cut a large rectangle of cardboard that is wider and taller than your cactus and fix aluminum foil to one side of it.
Stand the cardboard up so that the aluminum is facing the back of your cactus.
When you place a wall of aluminum foil behind your cactus it allows sunlight to bounce off the foil and reach more areas of your cactus.
And about once every week or two remember to turn your cactus so both sides are receiving an equal amount of light.
Doing this will help your cactus to grow more evenly and reduce the chance of it growing long spiny arms.
In addition to lack of light, keeping your cactus in an environment that’s too warm can also cause elongated growth and so can watering it too much during the winter.
Every cactus is different but as a general rule, you shouldn’t water your cactus more than one time every 4 to 6 weeks during the winter while keeping it in a cool 5 to 15 degrees Celsius environment.
One more issue that can cause your cactus to start growing arms is excess nitrogen.
Too much nitrogen, without a proper balance of other nutrients, may also cause your cactus to weaken and grow excessively.
You can use a soil testing kit to check nitrogen levels.
Also, check the nitrogen levels of any organic or chemical fertilizers before you use them on your cactus.
You can fertilize your cactus 2 or 3 times per year during the spring, summer, and fall, and that should easily satisfy its nutrient requirements.
Any quality houseplant fertilizer that’s diluted to half and higher in phosphorus than nitrogen should work well.
As long as you consider what your cactus needs from the start and ensure it is getting enough light, not too much water, the right type of nutrients, and a suitable temperature to grow, your cactus should not begin to grow arms.