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A calla flower is not a flower. The curved trumpet, white or yellow or deep plum, is a spathe: a single modified leaf that has taken on the job of attracting insects. The real flowers are the hundreds of tiny ones packed onto the yellow finger standing in the middle of it, which is called the spadix. The females sit at the bottom of that finger and the males above them.
This matters practically because it changes what you are seeing when a calla goes over. The spathe does not drop petals; it rolls closed, loses its color, turns green and then leathery, and the spadix inside starts to swell as the fertilized flowers at its base become seeds. That greening is not the plant dying and it is not a disease. It is the spathe returning to being a leaf, having finished the job it was made for.
It also explains why callas have no scent worth the name in most kinds, no petals to shatter across a table and an exceptionally long life in the vase: there is nothing delicate to lose. A cut spathe holds its shape for a week or two because it is built like foliage rather than like a petal.
The reasons a calla refuses to flower
A calla that grows well and does not flower is nearly always suffering one of a short list of causes, and they are distinguishable from one another by what the leaves are doing. Work through them in order rather than guessing, because the fixes are quite different and some of them require waiting a full season.
The most common by a distance is nitrogen. A calla in rich soil, or one fed a general-purpose fertilizer, builds a huge glossy clump of leaves and flowers barely at all. The second is shade: enough light to grow is not enough light to flower, and a plant under a tree or against a north wall will make leaves indefinitely. The third is a tuber or rhizome that is simply too small, which is what you get from a division cut too fine or from a plant whose foliage was removed early the previous year before it could rebuild.
The fourth applies only to the colored kinds and catches people out completely: a tuber that was not given a proper dry dormancy does not flower well the following year. Kept watered and warm all winter, it grows on tiredly without ever resetting, and it comes into the new season with nothing stored. The fix is to give it a real rest, which means a season lost but the plant recovered.
The fifth applies to the white kind: overcrowding. A clump left undisturbed for five or six years becomes a mass of competing rhizomes with all its energy in leaves. Lifting and dividing it restores flowering within a season.
| What the plant looks like | Most likely cause | What to do about it |
|---|---|---|
| Large lush dark leaves, one or two flowers | Too much nitrogen | Stop general feed; switch to high potassium; expect a normal year next season |
| Healthy but smallish leaves, no flowers | Too little light | Move to a spot with at least half a day of sun |
| Few small leaves, weak growth | Tuber or rhizome too small | Grow on for a season without dividing; feed after flowering time |
| Colored kind, second year, poor flowering | No dry dormancy last winter | Give a full dry rest this winter; flowering returns the year after |
| White kind, dense congested clump | Overcrowded after several years | Lift and divide, replant single crowns 18 in (45 cm) apart |
| Buds form then blacken and fail | Cold nights or bacterial soft rot | Check the tuber; protect from cold below 50F (10C) |
| Flowers small and short-stemmed | Dry at the root during bud formation | Water more consistently; the white kind especially |
Encouraging a good flush and keeping it going
Callas flower over several weeks rather than all at once, sending up spathes in succession from the crown while the leaves keep growing around them. The number you get depends mostly on the size of the storage organ, which was decided last year, and on how consistently the plant is watered and fed while the buds are forming inside the clump.
Two practical habits raise the count. The first is high-potassium feeding from the point the leaves are established until a few weeks after the last flower. The second is removing spent spathes promptly, since a calla left to set seed diverts a real share of its resources into the seed head and generally stops producing new spathes sooner.
The other lever is heat. The colored kinds flower faster and more freely in warm conditions and stall below about 55F (13C), which is why potted plants started early under cover often out-flower the same tubers planted directly outdoors. The white kind is the reverse: it flowers most freely in cool conditions with wet feet and often pauses through the hottest part of summer, resuming when the nights cool.
- Cut spent spathes right down at the base Once a spathe has rolled shut and turned green, follow its stem down into the clump and cut it at the bottom, not partway up. A cut stub left standing rots back into the crown and is a way in for soft rot. Wear gloves; the sap irritates skin.
- Do not remove leaves along with the flower stems Every leaf left standing is building next year's tuber. Deadheading is a matter of taking flower stems only, and a plant stripped of foliage while tidying flowers is the reason many callas flower once brilliantly and then decline.
- Cut for the vase when the spathe has just unfurled Pull the stem out of the clump with a firm steady twist rather than cutting it, which is how commercial growers harvest, or cut cleanly at the base. Take it when the spathe has opened but the spadix inside is still fresh and yellow, and stand it in deep cool water; it will hold for one to two weeks.
- Keep water steady while buds are forming A dry spell during bud formation gives short-stemmed undersized spathes, and no amount of watering afterwards adds size back. This is the moment in the calla year when consistent moisture converts directly into flower quality.
- Feed for four weeks after the last flower The flowering is over but the year's real work is not. This is the period in which the storage organ is rebuilt, and it decides how many flowers you get next year far more than anything you do next spring.
If you want seed, leave one spathe on the plant and let the spadix swell into a cluster of orange berries. Everything about those berries is poisonous, they take a long time to ripen at the plant's expense, and seedlings take three or four years to reach flowering size. For almost every gardener, dividing is the better route and the seed heads are best removed.
What to record about flowering
Note the date the first spathe opened, the total number of spathes each clump produced, and the date the last one went over. Flower count is the clearest single measure of how well a calla was treated the previous autumn, and comparing this year's count against last year's tells you plainly whether the plant is gaining or losing ground.
Record which plants you deadheaded and which you let set seed, if you tried both, and record the feed you used. A calla given general fertilizer one year and high-potassium the next will show the difference in flower count so clearly that a written note settles the question permanently for your own garden.