As an Arizona horticulturist I see a lot of succulents. Usually I’m planting them, and sometimes I’m just admiring the run-away growth of a mature cactus garden. Sometimes I’m called to help with cactus houseplants, and there’s a situation I see time and time again.
A frustrated homeowner admits sheepishly that it doesn’t seem to matter how they water, where they put the pot, how much light it gets, or what kind of cactus it is. Within a year or two it rots at the base and dies.
How is this possible? Are they screwing up the care instructions? Are most people just unqualified to take care of cactus? Are cacti really that tricky to grow??
Of course not!!
They’re actually just missing an important piece of the puzzle. They’re not doing a single thing wrong- and yet their cactus is doomed to fail. How is this happening?
It’s not your fault! It’s the soil
Their succulent was planted in a peat-based potting soil. Yes, reliable old potting soil. Time and time again, when i see a potted cactus rotting at the base, the problem is with the soil and not with anything the customer is doing.
The key here is that peat moss is excellent at water retention. Too good at it. Cacti need to dry out between waterings. Peat holds the water much longer than other ingredients- like coconut coir, decomposed granite, or forest products. By the time the soil dries the roots have already been damaged by the water trapped around them.
This is especially true for indoor cacti because houseplants tend to dry out slower than outdoor plants. Plus there’s usually less light available indoors, so the cactus doesn’t have enough energy to use all the water it’s receiving.
So maybe the owner is doing everything right and watering thoroughly twice a month or something like that. Maybe they have the cactus in a nice sunny corner and sing to it every day. But because the peat doesn’t want to dry out, the cactus suffers and eventually succumbs.
This is enough to make many people think they’re destined to kill cacti! But it’s not true, and there’s an easy solution for this problem.
How do you know if the soil is peat-based?
Peat looks like tiny threads and has a spongy consistency. It is dark brown and dense. When it dries out, it shrinks.
You can test the drainage of the soil by putting a sample of it in a small flowerpot. Add water and watch how long it takes for the water to flow out the bottom.
Lots of water staying inside the soil is a sign of bad drainage. You want cactus soil that allows the water to flow through the pot within seconds instead of minutes.
So you’ve determined it is peat moss
Time to repot! Preferably before the plant starts rotting.
Many desert cacti love to be planted in mineral based potting soils. Look for bags with decomposed granite and clay as ingredients. There should be lots of visible perlite or pumice at least.
Sometimes people sell combination “cactus and palm” soil mixes but i don’t really recommend them. Most palms love water and organic material and most cacti love rocky earth and fast drainage. By combining the two, you get a soil that isn’t great for either of them.
Try something like Black Gold or EB Stone’s cactus mix. Anything that seems very mineral-y. Sometimes i buy a peat-free potting mix and just add a lot of perlite, pumice, lava sand, decomposed granite, etc to make it a cactus mix.
What if my cactus is rotting already?
By the way, if your cactus is rotting at the base it’s not necessarily doomed. You can take a cutting by slicing pieces of the cactus above the rotting area. Leave them to dry for a while, then set them on/in some soil. Eventually you’ll have a cactus with brand new roots again. Worse case scenario you might just need to learn better for next time.
And take that peat moss and plant some flowers in it instead. It’s unsustainable anyway. Peat take thousands of years to develop and so it’s not renewable on any human timescale. It’s also too acid for many cacti, as if you need another reason not to use it!
Still not sure what’s going on?
I’m available for video sessions! You can ask me about a specific cactus or we can do a virtual walkthrough of your houseplants and garden. I can help with many succulents, orchids, tropical plants, and garden vegetables. I’m a certified nursery professional in the state of Arizona and i have years of landscaping and gardening experience.
Make an appointment here:
Paige Maloney | Arizona Landscape Designer 
Leave a comment