Sun, water, soil, pots

Four things decide how your cactus does: light, water, soil and pot. They are one system, not four separate settings — turn one up and the others have to move with it. Here are the numbers for each, and how they move together.

The rule that governs all four is that they interact. A well-watered, well-fed plant takes far more sun than a dry, hungry one. Stress from two directions at once is much worse than twice as much of one. So change one thing at a time and give the plant a week to answer.

How much sun is a question about your sky, not about the plant. Under mild northern light, most clones — what Clone means take all-day sun and hold good colour. Under high-UV Mediterranean or desert sun, the same clones go yellow on the all-day dose, and a few hours of direct sun is the better deal. The proof is that one collection, moved between exactly those two climates, changed its answer within a season.

What never changes anywhere: every change in light has to be gradual. One week's swing from cloudy and 80°F to sunny and 95°F is enough to scar a plant permanently. A mail-ordered plant has just spent one to two weeks in a dark box, so give it shade with plenty of ambient light for a few days, then half sun, then full.

Some numbers to work from. Shade cloth of 30 to 40% across the worst of the afternoon — roughly half past two until half past five — is enough to prevent sunburn — what Sunburn means. Indoors, aim for at least 750 PPFD — what PPFD means from a proper panel, and 1,000 to 1,250 if you can manage it; a bright window and a cheap grow light are not enough for a mature plant.

Then read the colour. Green through to blueish is healthy. Yellow, or red in high summer, is too much. Pale thin growth is too little. Tolerance varies from clone to clone, so two plants side by side can answer the same sun differently.

Soak the whole pot, let it dry out, soak it again — and test with a finger a couple of knuckles down rather than by the calendar. In real heat you can water before it is bone dry; when highs sit only in the 60s, let it stand dry for a few days first. Let the soil go dry before forecast highs drop under 60°F, or nights under 35°F, because cold and wet together is far more dangerous than cold on its own.

Both extremes do damage, and the dry one surprises people. Leave a pot bone dry for weeks and the fine root hairs die back, leaving small wounds; the water you then pour in gets into those wounds, and that is the rot — what Rot means people blame on watering. Never leave a pot standing in a saucer.

Cadence is a climate question, not a rule: roughly weekly per plant through a Californian growing season, twice a week through a Benelux one. Aim for about 6 to 10% of the water to run out the bottom, and at least 15% when you are feeding, so salts flush through instead of building up.

The opposite problem has a name. Oedema — what Humidity blister (oedema) means, which growers call humidity blisters, is raised bumps that appear when the roots push in more water than the skin — what Epidermis means can breathe back out. It comes from wetness plus poor airflow rather than from any infection.

Rich and fast-draining, in that order of surprise. A good starting ratio is about 65:35 mineral to organic — what Organic (in a soil mix) means, and the minimum-effort version of that is two parts pumice — what Pumice means to one part peat-free garden soil. Push toward 75:25 for bridgesii — what Bridgesii means, scopulicola — what Scopulicola ("scop") means and monstrose — what Monstrose means plants, and further again in a wet climate. A widely used alternative is half potting soil, a quarter perlite — what Perlite means and a quarter worm castings.

Particle size matters more than people expect: around an eighth of an inch for seed, three eighths for adults, and nothing over half an inch.

Three ingredients deserve a straight word. Perlite works and half the hobby uses it, but it floats to the surface over time and holds more water than it looks like it should; pumice does the same job without either habit, so use pumice when you can get it. Peat holds water longest of anything on this page — whole mixes are built on it in dry climates, and it is the first thing to take out in a wet one. The organic share is a lever, not a law: mineral-heavy mixes are the forgiving end because they starve fungus of standing moisture, while richer soil grows a faster plant where drainage and airflow genuinely keep up. Two feet of growth in a season has been done in mostly organic beds — by people whose drainage earned it.

Drainage holes, no attached saucer, and the plant no taller than the pot is wide or it will topple. Fill the soil to about half an inch below the rim — that gap is your watering reservoir. Glazed decorative pots with no hole are the one type to refuse outright; if there is a plastic liner inside, use the liner.

The usual ladder runs 2.7-inch cells up to about eight inches tall, then half-gallon pots from eight to sixteen inches, then three to five gallons as a permanent home. Leaving a plant too long in an undersized pot visibly slows it.

Materials trade off against each other. Plastic is stable and dries slowly, which suits a very dry climate, but it can bake the root zone in direct sun. Fabric dries fast and prunes the roots with air, but it destabilises when you move it. Terracotta wicks moisture and suits the wettest climates, but it costs most and the roots cling to the walls.

Full-speed growth happens with daily highs between about 68 and 86°F. Above roughly 95°F growth slows and then halts altogether — that is genuine heat dormancy — what Dormancy means, and the right answer is shade and no feed rather than more water. Below about 60°F highs, growth stops from the other direction and dormancy begins, with roughly 55°F as the trigger.

If your winter is harder than the plant can take outdoors, overwinter it somewhere between 35 and 55°F and dry — it will survive the season without any light at all. Dry is the word doing the work in that sentence.

Do not read that as the plant needing a dark sulk to flower. In Southern California, where winter barely happens, the same plants flower every year without anything resembling a rest. What they do seem to need is light: adults overwintered in complete darkness have sat out the following flowering season entirely. So overwinter them cool and dry by all means — just not in a windowless corner.