How to Grow Cacti From Seed

Sterilize Your Soil!

Next, before you start adding seeds to the soil, you need to sterilize it! It’s hard to make out in the photo above, but I have all of my pots arranged in a plastic nursery tray. I get a batch of these plastic growing trays every couple years and they work perfectly with both 2″ and 4″ square pots. The tray needs to have a solid bottom so the water sits in it! 

Some growers do this *before* they add the soil to the pots; I am a little lazier. I boil a large pot of water, get it to a rolling boil, then carefully pour it in each pot to saturate the soil. This immediately disrupts the thin layer of soil on top, so using my little Bonsai Rake, I just even out the surface again after I’ve poured the water. It’s okay if the surface isn’t completely, perfectly flat, as long as there’s some of the rich soil that the seeds can grab onto.

After this, leave it to sit and cool off for at least 5 to 10 minutes, preferably longer. The hot water sitting at the bottom will continue to heat up and steam the soil, and that should kill off any bugs or fungus. 

When you’re ready to start sowing, the pots should be comfortable to pick up and hold, and preferably be around 80 to 90F.  While the soil cools down from being sterilized, begin preparing your seed sowing station. 

Seed Sowing Station

Because this can make a mess and you’ll be dealing with very wet, recently saturated pots, it can help to put a towel down where you’ll be working. I use an old dog towel folded up so the water doesn’t pool on our counter.

I also  have a few items set next to me that are dedicated for just seed sowing. Anything that might also be a kitchen instrument I keep separately and labeled for the plants so I don’t have to worry about getting dirt and debris in the kitchen drawers.

I keep a box of Cling Wrap – basically, the non-sticky plastic wrap – next to me.

I also have tape handy; I cut up strips of packing tape because it’s what I have loads of, but scotch tape works well. If anything, scotch tape is better because it’s less sticky and won’t damage your pots when you take it off later.

PLANT LABELS are critically important. You will not remember what you planted or when. Write on the label using a pencil, not a pen or sharpie. Black markets and pens fade in sunlight, often faster than you expect, but pencil sticks around.

I like to have a set of tweezers handy, with this set being the exact set I’ve got for seeds and in the greenhouse for plant care. You can use them to place seeds precisely if you need to, rearrange things, etc.

A cup with play sand or just desert sand and something small to scoop it with is also needed.

Last but not least, a small squirt bottle with hydrogen peroxide. Any kind of bottle will do as long as it can make a fine mist; you’ll be using this as a final sterilization step.

It can seem like a lot, and it honestly sort of is, but for the number of seeds I sow this works very well. Some growers use plastic bags instead of my cling-film method, but I always have issues with mold, funk, or poor ventilation. 

Why the cling film? 

The reason you need to add the cling film on top is to keep humidity high for the seedlings. This is far from the only way to do so! There’s another grower in my neighborhood who sees a ton of success just using a sandwich bag to seal up his freshly sown seeds, opening it up from time to time to check on them or add water. 

I’ve heard of other growers just putting a pane of glass or plexiglass over a full tray of seeds, which would be ideal if I had to choose. It would just require some craftsmanship to build the case for it, and I am not quite that handy. 

I’ve tried using those little 6-cell garden seed starter kits, but found them to not keep humidity high enough and to be extremely finicky to work with and transplant seedlings. I’d notice seedling roots coming out of the bottom of the trays before the seedling bodies were big enough to support being transplanted, which is a frustration of such a short germination tray. 

The pots I use are 4″ square, but I’ve also had decent success with little 2″ square pots. 

How Long to Keep Seeds Covered? 

Once the seeds are sown, you should place them somewhere very bright but not in direct afternoon sun. I keep mine in a corner with east and north facing windows, so they get morning sun that fades to shade by noon. 

I also use a seedling heat mat to encourage germination, as the heat definitely seems to encourage them to pop up much faster than without. 

As for how long that should be…anywhere from a few days to a month! It depends entirely on the species you’re growing. This is one of the reasons I recommend Astrophytum capricorne or Astrophytum myriostigma as great candidates to learn with: they should germinate within a week at most. Their seeds are nice and big, easy to see, and it’s really easy to see when the tiny first plant bodies appear as little green nubs. 

You’ll want to keep the seeds covered for at least a few more days. Check your tray and keep the water topped up so the soil doesn’t dry out. With the seedling mat, they should be warm enough that you consistently see condensation on the cling film, which you can gently tap to see inside if anything has germinated. 

I eyeball my cacti to determine when they should be aired out. Once the plant bodies plump up a bit, and they at least double (preferably triple) in size compared to when they first germinated, that’s usually when I remove the cling film. For most of my cacti, that’s within the first week, sometimes two. I take the cling film off quite early compared to other growers I know, but as I keep them in a tray with water, they tend to not dry out too bad. If you’re really worried, a quick mist with a squirt bottle keeps the top of the soil moist.