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How these cacti flower

A gray smudge for weeks, then a white flower as long as your forearm. It opens after dark and is done by the next evening. What triggers it, the three bud stages, and why some plants never flower at all.

30-second version first, the whole guide below5 min read

The short version

  • The flower is the biggest thing this plant ever does, and you get about a day of it.
  • It opens after dark, white, up to a forearm long.
  • Flowers come from the areoles near the tip.
  • What carries a bud to an open flower is steady moisture.
  • A dry spell while budded is, growers say, the commonest reason buds drop, so water a budded plant more often than usual, not more heavily.
  • An unpollinated flower withers and drops whole, base included.

The flower, in six pictures.

Picture 1 of 6. Spines, branches and flowers all start from the same spot, growers say: the small felted pad under the notch on each rib.

The cutting says: Meh.

Picture 2 of 6. A white or gray tuft at an areole. Do not get excited.
Picture 3 of 6. The tuft stretches and green shows at its base. Now it is a flower, and it will almost certainly open.
Picture 4 of 6. The outer petals pull away from the bud. You are a few days out.
Picture 5 of 6. It opens at nightfall and is spent by the end of the next day.
Picture 6 of 6. By the next evening it is a wet rag hanging off the rib.

Your setup

Pick yours to see what changes in this guide.

Where it grows

In the ground

Ground plants bloom earlier and more freely, growers say.

In a pot

A root-bound pot can bring flowering forward too, but it costs you growth.

Growers report that a two-foot piece, three inches thick, has reflowered reliably when cut and repotted at that size.

Where it lives

Outdoors

Early warmth matters more than people expect.

One grower reports a collection moved to a colder climate losing roughly half its flower tufts after a cold May.

Indoors

Leaving a budded cutting in a dark garage and expecting it to open.

The season

Winter

Some hybrids ignore the calendar — one clone reportedly blooms right through winter.

You do not need a cold dormancy either, growers report: plants in climates where winter barely registers bloom every year.

Not winter

Late spring into summer, growers say, often with a second flush in high summer.

A dry spell while budded is, growers say, the commonest reason buds drop, so water a budded plant more often than usual, not more heavily.

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

The flower is the biggest thing this plant ever does, and you get about a day of it. It opens after dark, white, up to a forearm long. By the next evening it is a wet rag hanging off the rib. All pollen work has to fit that window, so, growers say, breeders read buds weeks ahead.

It comes out of an areole, like everything else

Spines, branches and flowers all start from the same spot, growers say: the small felted pad under the notch on each rib. Flowers come from the areoles near the tip. Two to five at once is common, and sometimes you get eight.

One areole, magnified

spines and a bud, one pad

Where they show up

upper stem · light side

A flower does not get its own organ. It comes out of an areole — the same felted pad that made the spines beside it, and the same one a pup would come out of. On a column the buds appear near the top, on the side that gets the light.One stem carrying buds several weeks apart is normal, and it is why a plant can flower more than once in a season.

The flower is a long cone rather than a shallow cup, usually 6 to 12 inches (15 to 30 centimeters) from base to petal tip, botanists report. White, sometimes yellow, rarely red. The tube is so hairy the bud looks felted. That hair named the genus. A hairy bud with a hairy fruit behind it is, growers say, the quickest proof of a true , not a lookalike.

A long cone rather than a shallow cup, and bigger than people expect: usually 15 to 30 centimeters from the base of the tube to the tip of a petal, carried out sideways from the stem rather than standing up off the top.The scale runs the whole length, base to petal tip. The swollen end where the tube leaves the areole is the part that becomes the fruit.
One flower, cut down the middle. The pollen is on the anthers, ringing the throat; the stigma is the green crown on the end of the style, standing clear above them.

Reading a bud: three stages

  1. Step 1: Stage one: a fuzzy dot

    A white or gray tuft at an . Do not get excited. Plants throw far more of these than they can carry; the surplus dries up and drops.

  2. Step 2: Stage two: it lengthens, and green shows

    The tuft stretches and green shows at its base. Now it is a flower, and it will almost certainly open.

  3. Step 3: Stage three: the petals lift

    The outer petals pull away from the bud. You are a few days out. Start checking in the evening.

Stage one — a fuzzy dot

felt and hair only

Stage two — it lengthens, and green shows

scales visible under the hair

Stage three — the petals lift

a few days out · check each evening

Three stages, and the first one is ambiguous — a fuzzy dot on an areole could still turn out to be a pup. It stops being ambiguous once it lengthens and green scale shows between the hair.Drawn at one magnification, so the growth between the panels is the real thing and not the zoom changing.

This one will open

The fuzz has stretched past a centimeter and shows green at the base. The plant has committed.

A plant throws far more buds than it can afford to carry and then edits itself back down. The one that is going to open has stretched out past a centimeter with green showing at its base; the one that is not stays a flat gray-white smudge on the areole, unchanged for two weeks. The two are drawn on the same stem, at the same areole, two weeks apart. A rooting cutting drops more of them than an established plant does — it is living off stored water and has less to spend.

A cutting does the same math. With more buds than stored energy, it reportedly aborts some rather than run itself flat. That is normal, not a fault. If you would rather have two flowers than six, pull the extras off with your fingers, growers say. The plant will not mind.

One night, and then it is over

It opens at nightfall and is spent by the end of the next day. Pollen is only worth collecting for a few hours of that: at dusk with the flower fully open, or at first light next morning. Rain or heavy dew ruins the pollen for that day.

Dusk — still shut

tips parting

After dark — fully open

one night only

Next afternoon — spent

closed · limp · will not reopen

It opens after dark and it is spent by the end of the next day. That single night is the whole window: for the flower, for an insect, and for you with a brush.Pollen is only worth collecting for a few hours around the opening. By the third panel there is nothing left to take.

That is why breeding is a bookkeeping job. Two plants you want to have to flower the same night, growers say, unless you already have pollen from one of them in the freezer.

What actually makes a plant bloom

What carries a bud to an open flower is steady moisture. Not floods. Steady. A dry spell while budded is, growers say, the commonest reason buds drop, so water a budded plant more often than usual, not more heavily.

Early warmth matters more than people expect. One grower reports a collection moved to a colder climate losing roughly half its flower tufts after a cold May. The tufts appeared, then never became buds.

  • Size is not the gate. Small plants flower. Growers report that a two-foot piece, three inches thick, has reflowered reliably when cut and repotted at that size.
  • A branch that has flowered will, one grower says, very likely flower again, even cut off and potted. Look for old flower scars when you buy.
  • from a mature stand flower sooner than cuttings from a young plant, growers report. The wood is old enough already.
  • In the ground beats a pot. Ground plants bloom earlier and more freely, growers say. A root-bound pot can bring flowering forward too, but it costs you growth.
  • Darkness cancels it. Buds on a cutting kept dark for two weeks never opened at all, one grower reports.

The season

Late spring into summer, growers say, often with a second flush in high summer. Some ignore the calendar — one reportedly blooms right through winter. You do not need a cold either, growers report: plants in climates where winter barely registers bloom every year.

After the flower

An areole that has flowered can flower again from the same spot. The only way to lose that spot, growers say, is tearing the fruit off by hand and pulling the growing point out with it. Cut the fruit off at its base with a blade instead.

An unpollinated flower withers and drops whole, base included. Nothing is left behind. What happens when it is pollinated is two guides further on. The guide in between explains why the plant next to it cannot do the job.

It took

base swelling

It did not

base dry · flower shed

About two weeks after the flower closes you know. A pollinated flower thickens at its base into a fruit; an unpollinated one dries at the base and falls off whole.
Common mistakes, try it, quick check and sources

Quick check

  1. Question 1: When do these flowers open?

  2. Question 2: A plant is budded and a dry week is coming. What do you do?

  3. Question 3: A small gray fuzzy tuft has appeared at an areole. What is it?

Ran into a word you did not know? The glossary has the common ones, and the questions page covers the ones people ask most.