That little cactus you got when you were younger, well, yeah, maybe it hasn’t been doing much all these years. You didn’t realize what size it would eventually gets (or you did), and got the wrong thing. Either way, cacti tend to be slow growers and need some room, so you should of plan where they will go before you plant them.
As the chart indicates, each species ends up quite different sized. Check the chart data before buying your plant. You don’t want to think you have a giant cactus for your landscape, only to find you have something miniature. The reverse can also happen.
Check the Size Before You Buy
That tiny-looking young saguaro looks like a suitable shrub-sized plant, but it will take decades to become true giant. Keep in mind how big your cactus wants to be, and you’ll save yourself the heartbreak of sticking a big guy into a small pot.
Barrel types versus columnar giants: While some species, such as the Saguaro and Cardon, get to be sixty feet tall in the wild (meaning that these must is planted in frost-free conditions), pincushion varieties like mammillaria stays under six inches all their life. This is biologically how they’re made to live, true desk dwellers, fitting on a shelf. You are fighting against their nature if you try to make a mini cactus of any kind into floor plant.
And don’t forget: size includes width, something most novices overlook. Some golden barrel cacti is just three feet high, yet they span thirty-six inches in width. They’re spheres, not columns. So examine both its form and how wide its stem gets. You don’t want that thing taking down your neighbor’s (or crashing through your) curtains if all you consider is height.
The plant is your guest; you choose the space and the type of plant. Growing them inside makes things even harder: most pots only hold plants up to four feet tall, so you can only grow the youngest columnar and barrel cacti. And even then, you’d have to provide very serious light (you can’t match the needs of a sixty-footer indoors). Trying to sustain a full-grown saguaro indoors will be a losing battle. Accept it: Some cacti are not made for livig inside.
And then there’s the issue of height: The taller the plant, the slower it will seem to move. You’ll get instant gratification if your plant (like a cactus on your desk) adds an inch a year or even more; you will see the results. But a columnar that has been growing in the landscape for years will only gain maybe half an inch each year, and appear still to human eyesight. This is normal biology. Adjust your expectations to the plant’s biological clock.
That’s the thing with choosing a cactus: You select for its eventual shape, not merely for its beauty today. Measure the dimensions of your available area. How big are your windows? How high is ceiling? The excitement grows when you match a plant to its space. That little seedling isn’t stuck; rather, it’s exactly what it was intended to be.