Growing Cacti from Seed
Note: Every cactus specie is different in its requirements for growing. This guide is just a general reference that works great for *most* cactus seeds.
Materials: This is what you will need.
* spray bottle for water
* fine sharp sand
* lights (fluorescent are good)
* plastic bags/plastic bottle (humidity tent)
* warm area
Soil: Every cactus grower seems to have their own "special" mix. Everyone seems to swear by their own mix. So, what one is right? The reason so many people have different mixes that work "best" is because it is suited to their climate and growing style. You must first think about your climate and how to work with it. In Canada we needed something that would dry because it was not too warm and we did not want things to stay wet. In Taiwan we use a very sharp well drained mix for things like Astrophytum but we add a little bit of soil to keep more moisture/nutrition for other species such as Lophopohora sp.
You want (and don't want) a few things in your soil. Make sure it is free draining. We want the water to flow right through without acting like a sponge. Ideally, a fully soaked pot will dry out in 1-3 days. Also avoid organic debris. Although in dry areas this is not a problem, but in places where the pots may remain wet this will invite fungus, bacteria and bugs (not good). Also try to avoid soils that are too rich. Plan to have a very nutrient deficient soil then add nutrition (in whatever form you choose) later. When using sand choose sharp sands that tend not to bind together. River and beach sands are generally poor choices. Check out gravel pits/yards to see what they have.
With seeds we prefer a straight sand setup. We then add slow release fertilizers later one when they are a little more established.
Getting started: First we want to get our pots, or whatever we are using for containers, ready. Anything will work, we prefer plastic for seedlings. Poke some small drainage holes in the bottom if there are not any already. Add some large pumice chunks or stones to the bottom, this will make sure the pot doesn't puddle when watering. We fill 3/4 of the pot with a very sandy/gritty cactus mix. We prefer not to have any organic debris in here. Some people like to mix in some coco coir, which can work in some climates. We then top dress the pot with about 1 cm of fine sharp sand. This is important so that the seeds do not fall down too deep into the substrate. See Soil Types for more information.
Sowing the seeds: Now that the pots are ready you are ready to put the seeds in. It is important not to burry the seeds too deep. In fact, we prefer to sow the seeds on the surface and leave them as is. Many cactus seeds that are buried too deep will not sprout and just die. For this reason, sowing on the surface is the best option. You may also sprinkle a *thin* layer of fine sharp sand over top if you feel the need, but not too much.
Watering and Humidity: Now that the seeds are placed on top of the fine sand it is time to water. We like to use a fine mist spray bottle to do this as it does not disrupt the seeds and waters the surface evenly without puddles. Give the seeds a good soak so that most of the substrate is moist. At this point in a cactus' life they are actually not good with dry conditions. Most species benefit from very high humidity and do not like to dry out. After watering place the pots inside a zip lock bag or place a cut bottle or clear plastic cup over the pot to create a humid environment, while still allowing light in. Depending on species, keep the seedlings in the humid environment for 1-6 months.
Lighting: Again, seedlings are not like bigger cacti. They do not like overly dry conditions, nor do they like super bright conditions. For the first while it is best to keep them inside under weak artificial lighting. We like to use fluorescent bulbs (tube and CFL). We often use 2x 24" daylight (6500k) tubes for seedlings. Sometimes seedlings will turn red, this is because there is too much light and your cactus just got a sun burn....yes, cacti get sun burns!
When the cacti get larger you can increase the light. They will tell you when they need more or less. If you notice your seedlings becoming tall and "stretched" looking, then you need more light. They are doing this because they are looking for light. If they are red, reduce the light or put up some kind of light filter (such as glass or ceran wrap).
Heat: Keeping the temps in the 20-30 degrees Celsius range is best. Try not to allow the temps to fluctuate too much.
Puberty and Acclimation: Depending on species, your cacti seedlings may be ready to go outside in 3-18 months. We generally don��t put anything outside until it is at least 0.5 cm. No matter the size, when moving your cacti outside it is important to do it slowly. Taking a cactus from a darker indoor setup to real sun is a huge shock that could very well kill the young cactus. We often acclimate them over a 2 week period.
Too moist: If the substrate is kept too moist (wet), rot may set in. The ideal moisture for many cacti when germinating and the first few months is to have the soil slightly moist, but no where near wet. Always keep it just a little more moist than when the very surface begins to dry out.
Too dry: Cacti seedlings are so tiny and thin that they are not yet able to retain water well in any quantity. This is the reason why humid conditions are favoured over dry conditions. Completely dry substrate will lead to dehydration, followed by death, often within days. The younger the seedlings the more sensitive they are, in general.
No air flow: Lack of air flow can be ok at times or it can wipe out an entire pot/tray. Lack of air flow allows a few tings to happen: over heat, become very wet and the moisture in the air will condense and make things soggy, and mold will spread. Mold is often the most common problem among seedlings. In general seedlings are *fairly* (not always) resistant to fungus. But in times of stress or non optimal conditions fungus can wipe out a plant. Even just lifting the humidity tent/dome (whatever you are using) once a day to check is often enough. Poking a couple small holes will help with this as well, but make sure not too make too many/large as to loose all the humidity.
Green substrate: Green is algae. Algae is not usually anything to worry about as far as seedling health goes. But it is a good indicator of moist soil (a little drier is probably ideal) and nutrients available (fertilizers or perhaps organic debris). One thing algae may do is make the surface hard on top, which doesnt allow water and air to pass freely through the substrate. One other potential problem is when the algae dies, it is an invitation to fungus and bugs.
"cobwebs" and mold: Fungus invasion. This is often caused by available organic nutrients, too much moisture and not enough air flow. The best way to prevent this is not use any organic based mixes (like coco coir, peat, little wood bits that are often in commercial mixes etc). Increasing air flow and letting the area dry out a little are also good ideas. Fungus gone unchecked may wipe everything out, if corrected and the seedlings are in good condition, it should not be a problem if its only a temporary thing.
Pests: Due to the availability of water and little bite size snacks we call cacti, seedling trays are a welcome source of food for many critters. Do what you can to prevent them. The following pests may be a problem to cacti seedlings: Fungus gnats, caterpillars, cockroaches, snails/slugs, mice/rats, various soil mites, various fly larvae (species unknown).
Brown-red seedlings: This is usually from too much light. Lower the light levels. Low temperatures and/or lack of water also may cause colour change. Dont confuse this with rot.
Purple seedlings: This is often caused by lack of water. Some species may go some kind of "purple" shade under too much light and/or cooler temperatures, but often is from not enough water. Older plants may change purple from lack of water as well.
Splitting: Not a common problem with seed grown plants, but some slow growing species grown under "seedling conditions" and given too much water/nutrients may split as they bloat too much for their skin to handle. This often happens with some Astrophytum plants when given too much water and fertilizer.