Question
I just read your wonderful answer to a reader about calla lilies, and I am so impressed by the depth of your knowledge, your willingness to take the time to provide it, and the thoroughness of it. I do have one additional question. I have searched for this information on various plant sites, but putting in 慿eeping a calla dormant" led me to you.

I bought several Crystal Blush callas from Brett and Becky抯 bulbs, and decided to experiment with forcing them onto dormancy over the winter.  I completely withheld water and put them in my heated basement (heated, yes, but in zone 5a) and on March 22 (I keep excessively detailed records) I noticed that several growing points had risen above the soil line.  They may have been responding to the increased light, as the days grew longer in spring. I panicked and potted them up, put in terra soc water holding crystals, time release fertilizer and by April 18 they had grown substantially and began to unfurl huge leaves.  Unfortunately I could not put them outside for more than a six weeks (our last frost date is May 15), and they became quite unwieldy by May 28, when temperatures rose above 55.  I actually had to chop leaves off and stake them.  They bloomed anyway, and the chopped leaves are growing back but they have less than their usual charm, particularly with bamboo stakes holding them up.

My question: how could I retain the dormant period longer?  I actually have a little seed refrigerator in the basement (holds them ay 40 degrees), but it is too small to accommodate the pots.  Should I leave them in the pots somehow, or could I tale plastic bags of soil and put the callas in them in the small refrigerator.  If I did that would it be necessary to leave the bags open so that they can 揵reathe?

Thank you so much!  (I agree with you about WFF).

Donna Mack
Zone 5a gardener
Grayslake, Illinois

By the way, my favorites are ornamental grasses, especially miscanthus, lilies, peonies, lilacs, and old garden roses.

Answer
Hello Dr. Donna - This is a tough question. I had to do a little scientific research to understand the gymnastics that were going on downstairs with your Calla Lilies.

You provided extensive details - my guess is you have a PhD in SOMETHING - and in this case, different cultivars are very likely to react differently to an environment.  I have some specific information about Crystal Blush, but some of the following was obtained from reports on other cultivars; I am concerned that what I do know is POSSIBLY limited to the other cultivars.  So keep that in mind as you read.  It is not written in stone.  I apologize for that.  

The industry rule about storing dormant Calla rhizomes is 50 degrees F at 80% humidity.  Since the average American basement is frequently the most humid space in the house, air moisture levels would not be any problem for you.

It's the 50-degree-and-under temperature that's so hard to maintain perfectly.  Can you un-heat your heated basement?

I think THAT -- and not your lighting -- is where you ran into problems.

Calla varieties vary in their growth habits and needs, as I mentioned earlier.  Potted in the dead of winter and warmed to room temperature, your Crystal Blush Calla will begin to "flower" -- i.e., throw up its spathe -- about 9 weeks later.  Planted in spring, it will bloom in 8 weeks.

There are quicker-blooming, "early" Callas that flower in only 8 weeks.  And some that take 10 or 11 weeks before the first spathe opens.  Peak bloom takes another 1-2 weeks.

Smaller rhizomes are about 5 days slower than their grown up counterparts.  A small rhizome measures 2 1/4 inches or less.

Your 40 degree refrigerator might be ideal for these.  But this is where experience comes in.  I have not been there, I have not done that with Callas.  Given the tools, I would definitely give it a shot with some of your rhizomes, and stick to a single cultivar and rhizome size.  Since you are a careful record keeper this will probably be second nature to you.  One thing you absolutely should NOT do is put them in sealed plastic bags as this will breed anaerobic bacteria and you will most definitely have a bag of soggy mush at the end of the winter.  This time I'm talking from experience -- I HAVE been THERE, done THAT.

As you can imagine, growers who practice forcing their Callas in winter run into lots of fungus and disease -- problems that favor cool, wet environments.  If you manage to save all your tubers, I think you must have the greenest of thumbs not to have discovered an epidemic of tuber-rot in your Calla population.

After dormancy breaks, it seems that temperature controls the speed of the first flowering.  They need a minimum of 4000 footcandles at this point for healthy -- not optimal -- growth.  Good, strong sunlight and evening temperatures below 60 degrees F create stronger stems and richer colors.  Especially when it comes to pinks.  

Since these are not actual blooms being discussed, cultural requirements become interesting.  Callas don't need Phosphorous beyond the basic root-enhancing doses.  They do respond to Nitrogen feedings, a vitamin for strong, healthy leaves that we would usually not recommend for encouraging flowers.  But you also have to be careful not to overfeed with Nitrogen.  

Think of it as holding your wrist out and squeezing a baby bottle to check the temperature of the formula before you give it to the baby.  You just know what to do.

After flowering, Callas need to grow on for at least 10 weeks.  They need a period of dormancy for 8-10 weeks before re-growing.  Cool temperatures and reduced water initiate dormance.  

Callas are a profitable crop, and the industry is working hard to understand the mechanisms of flowering and dormancy.  Storage temperature and duration won't change color or flower size.  But the number of flowers changes with the length of dormant storage.  

Professional growers do a lot of weird things to their plants.  You know that phrase "Don't try this at home"?  As a business, all they worry about is the bottom line.  So they use things like Plant Growth Regulators to alter growth habits and genetic DNA.  This is serious stuff we're talking about.  Meanwhile, they really are not sure about what makes Callas tick.  As more people see them at the florist, more people will want to start growing them at home.  Inch by inch, we'll understand more.

Brett and Becky Heath, by the way, are the Gold Standard of the bulb business.  They have books out, they lecture, they are the ultimate experts -- but it for is their sterling character that I say that.  You are in good hands.

I have contacted several people I know with very strong understanding of these plants.  When I hear something useful from them, I will let you know more.  

Thank you Doctor for a really good question.