Gadenin日本語Get the appHow much light it actually needs
A desert cactus evolved under open sky, and indoors it is almost always short of light rather than burnt by it. The tissue it builds in poor light is soft, pale and stretched, and once a stem has grown that way it never firms up again. Aim for the sunniest window in the house and accept that anything less is a compromise you will see in the shape of the plant.
Four to six hours of direct sun through glass is a working minimum through the growing season. Less than that and growth turns narrow and pale; a globular plant starts to point, and a columnar one puts on a section noticeably thinner than the one below it.
| Position | Suitability | What you will see |
|---|---|---|
| Unshaded south-facing window sill | The best indoor spot | Dense spines, tight growth, flowers on mature plants |
| East or west window | Workable | Sound growth but slower. Turn the pot to keep it even |
| Three feet (1 m) back from a bright window | Marginal | Pale narrow new growth within a single season |
| North window or a room lit only by a lamp | Not suitable | Stretching, then collapse under its own weight |
| Outdoors in full sun, summer | Excellent once hardened off | Hard, well colored growth. Needs rain cover |
Move it into stronger sun gradually
A plant that has spent winter indoors behind glass will scorch if you carry it straight into full outdoor sun in spring. The damage is permanent: a bleached white or brown patch on the sun-facing side that stays there for the life of the stem.
- Start in bright shade Put it somewhere with open sky overhead but no direct beam for the first week outdoors, such as under the north wall of the house or beneath light mesh shade.
- Add morning sun In the second week give it direct sun until mid-morning and shade for the rest of the day. Morning sun is weaker and the plant is cool when it arrives.
- Go to full sun over two more weeks Extend the sunlit period by an hour or two every few days. Four weeks from the start it can take an unshaded position.
- Mark which side faced the light Put a small dab of paint or a tie on the pot rim on the side that faced the window, and keep that side towards the sun. A plant turned suddenly scorches on the face that was in shade.
Glass filters most ultraviolet, so a plant that looks perfectly happy on a sunny window sill is still unhardened as far as outdoor sun is concerned. Harden it off even if it has been in blazing light all winter.
Air movement matters as much as light
Still, humid air around the base of a cactus is what turns a small wound into rot. Outdoors this takes care of itself, but indoors a plant crowded on a windowsill between other pots sits in dead air all day.
Leave a hand's width of clear space around each pot, and open a window on mild days. If the room has no moving air at all, a small fan running on its lowest setting for a few hours a day makes a real difference to how quickly the compost surface dries.
The winter position is the important one
Most people get the summer right and the winter wrong. A desert cactus needs a cold, dry, bright rest through the coldest months, and without it a mature plant will simply never set flower buds no matter how well you treat it in summer.
The target is a bright place held between 41 and 50F (5 to 10C), with no water at all. An unheated porch, a spare bedroom with the radiator off, a bright garage or a frost-free greenhouse all work. A warm living room does not, because a plant kept at 68F (20C) stays half awake, keeps trying to grow in poor winter light, and produces the weak stretched section you find at the top of so many old collection plants.
| Winter room | Temperature | Result |
|---|---|---|
| Unheated bright porch or spare room | 41 to 50F (5 to 10C) | Proper rest. Buds form in spring |
| Frost-free greenhouse with a heater on low | Around 41F (5C) | Ideal, provided it stays completely dry |
| Heated living room | Around 68F (20C) | No rest, stretched growth, no flowers |
| Anywhere that freezes | Below 32F (0C) | Frost damage, which shows as collapse in spring |
Cold and wet together is fatal. A cactus at 41F (5C) in bone-dry compost is safe; the same plant at 41F (5C) in damp compost rots at the root plate within weeks.
Reading the plant for a bad position
The plant reports on its position more clearly than most houseplants, because the growth it makes is a permanent record of the light it had at the time.
| Sign | What it means | What to do |
|---|---|---|
| New growth narrower than the old body | Not enough light | Move to the brightest window, or outdoors in summer |
| Pale green, soft, widely spaced spines | Far too little light | Same, but harden it off slowly or it will scorch |
| A bleached white patch facing the sun | Sunburn | Shade it now. The scar is permanent |
| The whole plant leaning towards the window | Light from one side only | Give it a quarter turn every few weeks |
| Reddish or purple flush over the body | Sun stress or cold, usually harmless | Nothing, unless it is combined with softening |
Worth writing down
Note the date you first put the plant outdoors and the date you brought it back in, along with the coldest reading you saw in its winter room. Those three figures explain most of what happens the following spring, and after two or three years you will know exactly how early you can risk moving your own plants.
Also record which side of the pot faced the light. It is the single easiest thing to forget and the easiest way to scorch a plant you have owned for years.