Sun, water, soil, pots
Light, water, soil and pot are one connected system. The numbers for each, and what to change first when something looks off.
30-second version first, the whole guide below7 min read
The short version
- Light, water, soil and pot decide how your cactus does.
- Turn one dial and the others have to move with it.
- Every change in light must be gradual. One week's swing from cloudy and 80°F (27°C) to sunny and 95°F (35°C) can scar a plant for good.
- Test with a finger two knuckles down, not a calendar.
- For minimum effort, two parts pumice to one part peat-free garden soil.
- Rot follows soil that stays soggy, which is why a pot never stands in a saucer.
Your setup
Pick yours to see what changes in this guide.
Where it grows
In the ground
In the ground there is no pot to lift out of the rain, so the site has to drain: in heavy clay, mix in pumice, up to about a quarter by volume, or plant on a raised bed or a sunny slope.
In a pot
Rot follows soil that stays soggy, which is why a pot never stands in a saucer.
Where it lives
Outdoors
Under mild northern light, most clones take all-day sun and hold good color.
Under high-UV Mediterranean or desert sun, the same clones go yellow.
Indoors
A bright window plus a cheap grow light is not enough for a mature plant.
The season
Winter
If winter outside is more than the plant can take, keep it at 35 to 55°F (2 to 13°C) and dry; it survives the season with no light at all.
A pot can be carried under cover before a freeze; a plant in the ground can only be covered for the night, with cotton, burlap or frost cloth and never plastic.
Indoors, the winter rest is a cool, bright room: nights near 46 to 50°F (8 to 10°C) and almost no water.
Verne's Juul's Giant, from the cultivar library
Cold: reportedly it takes several days below 32°F (0°C) provided the soil stays dry, and some growers cover the growing tips during hard freezes in unheated greenhouses.
Not winter
In a hot, dry spell the mix dries fast: about weekly per plant, and sooner in a heat wave.
Lines are from this guide, or from the cultivar library where one says so. For where they come from, see the end of the guide.
Introduction
Light, water, soil and pot decide how your cactus does. Turn one dial and the others have to move with it.
Four dials, one rule
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 dial at a time and give the plant a week to answer.
Interactive
Set the dials
Pick a combination and see what it is likely to do over a season. Try the presets against each other: a summer outdoors, a shelf indoors, a greenhouse, a wet winter.
Season
Try a real place
Full sun all day, a proper soak whenever the pot has dried through, and a mix that lets it. In a hot dry summer that works out at about once a week for each plant.
Likely outcome
Fed and watered, it takes the sun
Most of these tolerate all-day sun outside the most UV-intense places, and a plant that is drinking and feeding well stays greener in it than a dry one. Growers who water and feed heavily shade their plants less, not more.
Fed and watered, it takes the sun. Most of these tolerate all-day sun outside the most UV-intense places, and a plant that is drinking and feeding well stays greener in it than a dry one. Growers who water and feed heavily shade their plants less, not more.
Light
How much sun depends on your sky, not the plant. Under mild northern light, most take all-day sun and hold good color. Under high-UV Mediterranean or desert sun, the same clones go yellow. A few hours of direct sun serves them better. One collection moved between those two climates changed its answer within a season.
Every change in light must be gradual. One week's swing from cloudy and 80°F (27°C) to sunny and 95°F (35°C) can scar a plant for good. A mail-ordered plant has just spent a few days in a dark box: shade with bright ambient light for a few days, then half sun, then full. A plant that has lived indoors, in a greenhouse or in winter shelter is in the same position when it goes outside: a few weeks in bright shade first, then more sun a little at a time.
- Burn risk peaks nearer midday and depends on how used to the sun a plant is. Shade cloth of 30 to 40% over the middle of the day helps prevent .
- Indoors: at least 750 from a proper panel, 1,000 to 1,250 if you can. A bright window plus a cheap grow light is not enough for a mature plant.
- Under glass, heat builds up. Open the vents on sunny days and shade the glass through the warm months, using only as much shade as the heat needs.
Then read the color. Tolerance varies by clone: neighbors can answer the same sun differently.
- Green through blueish: healthy.
- Yellow, or red in high summer, is too much sun.
- Pale, thin growth means too little.
Grown in full sun
short · fat · pale · tight areoles
Grown under shade
tall · slim · dark · areoles apart
Water
Soak the whole pot, let it dry, soak again. Test with a finger two knuckles down, not a calendar. In real heat, water before it is bone dry; with highs of 60 to 69°F (16 to 21°C), let it stand dry a few days first. Get the soil dry before forecast highs under 60°F (16°C) or nights under 35°F (2°C). Cold and wet together is far more dangerous than cold alone.
Too dry is not free either. Weeks bone dry and the plant sheds its fine root hairs; they regrow once it is watered. follows soil that stays soggy, which is why a pot never stands in a saucer.
How often depends on how fast the mix dries, growers say:
- In a hot, dry spell the mix dries fast: about weekly per plant, and sooner in a heat wave.
- In a cool or humid spell it dries slowly, so go by the finger test, not the date.
- Aim for 6 to 10% out the bottom, at least 15% when feeding, so salts flush through instead of building up.
, or humidity blisters: raised bumps that appear where roots push in more water than the can breathe out. Wet plus poor airflow — not infection.
Soil
Rich and fast-draining at the same time:
- Starting ratio: about 65:35 mineral to .
- For minimum effort, two parts to one part peat-free garden soil.
- With , and , push toward 75:25, and further again in a wet climate.
- A common alternative is half potting soil, a quarter , a quarter worm castings.
- In the ground there is no pot to lift out of the rain, so the site has to drain: in heavy clay, mix in pumice, up to about a quarter by volume, or plant on a raised bed or a sunny slope.
Particle size matters: around an eighth of an inch for seed, three eighths for adults, nothing over half an inch.
A drop or two
right
A stream
too wet
Falls apart
too dry
Figure
Where the water actually goes
Three pots in section. The first is the most repeated piece of bad advice this plant meets, and the reason it is wrong is entirely invisible from above.
Gravel in the bottom
makes it worse
Fine mix will not let go of its water into coarse air gaps until it is nearly saturated, so a wet band parks itself exactly where the roots are.
One mix, all the way down
what to do instead
Nothing for the water to stall against, so the front travels through and leaves. You also get the whole pot back as usable soil.
Left in a full saucer
undoes the rest
Whatever drained out simply wicks back up. The buried green skin ends up standing in it, which is where basal rot starts.
Perlite works, but it floats to the surface over time and holds more water than it looks. Pumice does the same job without either habit; use it when you can.
Peat holds water longest of anything here. Dry climates build whole mixes on it; in a wet one it is the first thing out.
The organic share is a lever, not a law. Mineral-heavy is the forgiving end — it starves fungus of standing moisture. Richer soil grows faster where drainage and airflow keep up: two feet in a season has been done in mostly organic beds, by growers whose drainage earned it.
Pots
Drainage holes, no attached saucer, and a pot heavy and wide enough that the plant does not topple; stake a tall column rather than buying a pot as wide as it is tall. Fill to about half an inch below the rim; that gap is your watering reservoir. Refuse glazed decorative pots with no hole. If one has a plastic liner inside, use that.
Pot size follows plant height:
- 2.7-inch cells: up to about eight inches tall.
- Half-gallon pots: eight to sixteen inches.
- Three to five gallons is the permanent home.
Too long in an undersized pot visibly slows a plant.
Cell
a rooted cutting
Half gallon
a year on
Five gallon
an established plant
- Plastic: stable, dries slowly, which suits a very dry climate. In direct sun it can bake the root zone.
- Fabric dries fast and air-prunes the roots, but destabilizes when you move it.
- Terracotta wicks moisture and suits the wettest climates. It costs most, and roots cling to the walls.
The seasonal rhythm
- Daily highs 68 to 86°F (20 to 30°C): full-speed growth.
- Above about 95°F (35°C) growth slows, then halts. That is heat : give shade and no feed, not more water.
- Growth slows when highs stay below about 60°F (16°C), and stops, with dormancy beginning, around 55°F (13°C).
If winter outside is more than the plant can take, keep it at 35 to 55°F (2 to 13°C) and dry; it survives the season with no light at all. Dry is the key word. A pot can be carried under cover before a freeze; a plant in the ground can only be covered for the night, with cotton, burlap or frost cloth and never plastic. A greenhouse only has to hold that range and stay dry. Indoors, the winter rest is a cool, bright room: nights near 46 to 50°F (8 to 10°C) and almost no water.
No dark rest is needed to flower: in Southern California, where winter barely happens, the same plants flower every year. Light is needed. Adults overwintered in complete darkness have sat out the next flowering season. Cool and dry, yes; a windowless corner, no.
Figure
When a dial is wrong, this is how you find out
Everything above is what to set. This is what the plant does when one of the four is off, and which sign belongs to which dial.
Sunburn
too much light, too fast
Yellow on the lit side only, worst on the rib crests. The two darker patches have scarred.
Etiolation
too little light
New growth thinner than the old, pale, fewer ribs, stunted spines, reaching sideways.
Rot at the base
too wet, too cold, too deep
Soft and dark, starting below the soil. The pale margin means it is still moving.
Edema
water in faster than it breathes out
Raised blisters on skin that is otherwise perfectly green. Wetness and still air, not an infection.
Common mistakes, try it, quick check and sources
Quick check
Question 1: Your plant has been in shade all winter and today is a bright 95°F (35°C). Do you move it into full sun?
Question 2: Should you put a layer of gravel in the bottom of the pot for drainage?
Question 3: It is 55°F (13°C) and forecast to rain all week. Do you water?
