Bedtime Stories

As we get ready to celebrate the season’s holidays, a good number among us are busy moving dahlia tubers into winter storage. Typically, these hibernate for a couple of months before awakening from their slumber to stretch dormant buds to greet the warming days of March. So “putting them to bed”, as I explain to others, is an apt expression. In dahlia circles, the choice of bedding material ranges widely – from shredded newspaper to peat moss, from bark dust to sawdust, from play sand to popular cedar shavings . . .the picks are plentiful. Successful storage by some encourages others to try even more esoteric mediums. Kitty litter or perlite, anyone? 

Yet the most enduring material for storing dahlias has been the mineral vermiculite. Having used it exclusively for the past 50 years, I can attest to its annual reuse and insulating qualities. In fact, I finally just had to buy a bag as my supply was running low, probably due to gradual loss every season in transferring vermiculite between storage tubs and tuber bags. “What? You still are using vermiculite?” Some of you recoil in horror, prophesying my inevitable demise. 

Well, let me explain. Some 25 years ago, a Seattle newspaper published an investigative series on the health crisis in a small Montana mining town. Miners and their families were afflicted by asbestosis, a chronic lung condition leading to cancer and caused by long-term exposure to high concentrations of asbestos fibers in their community. Libby, Montana became known as a death trap, and the old Libby mine - which mined vermiculite for W.R. Grace - was found to be the source of dangerous accumulations of these fibers, as rock containing vermiculite was processed by cooking it at high heat. Such rock contains sheets of silicate mica and some moisture; heating expands the mineral up to 30 times its original size. Upon investigation, it was found that the Libby mineral also included 30% tremolite asbestos fibers that was responsible for more than 400 deaths and several thousand documented asbestosis victims. Although the mining operations were halted in 1990, enough of that stuff ended up as insulation in America’s attics or as spray. The town became an EPA Superfund site; the W. R. Grace firm was sued successfully for compensation and had to declare bankruptcy in 2001. If you find Zonolite in your home today, it came from Libby, Montana, and requires professional asbestos abatement. 

These days vermiculite is mined elsewhere. Virginia and South Carolina now provide most commercially available vermiculite, although mining also takes place in South Africa, Russia, Brazil, and China. Aware of its potential health hazards, the American products are regularly inspected for contamination before being used in construction, thermal insulation, brake linings, packing, and for gardening. Mindful of its history, users are admonished to pour fresh vermiculite outdoors with proper eye protection and face mask to avoid inhaling dust. 

Vermiculite comes in four grades, from very fine A-1 to the coarsest, A-4. I learned that the finer (horticultural) grades used in potting soil retain too much moisture and would rot tubers in storage. But those are just the grades most often sold in garden centers. If you plan on using vermiculite to bed down your dahlias, get the coarse grind from a specialty store or online. It does not need to get wetted down, and the plastic bags I use have no perforations. Seldom is there any storage failure. It’s an insulation material, after all. 

Several advantages of vermiculite must be noted: it is a sterile mineral (so no fungus or disease potential). It is light-weight, of course, and it can be stored outdoors in tubs during the year. Vermiculite is reusable, may retain water, and attracts other plant nutrients, such as potassium, magnesium, and calcium. In time, vermiculite does break down, holding more moisture. Its aeration properties then makes it suitable in a potting soil mix and poses much less hazardous transfers.

What about perlite, then? Another mineral, perlite actually is obsidian volcanic glass that (when heated to 1,000 °C) pops into white porous grit. Normally used as soil amendment or starting medium, perlite is an expensive alternative to other dahlia beddings. While it is light-weight, holds moisture, and is sterile like vermiculite, breathing in fine perlite dust without a respirator allows it to settle in one’s lungs, causing airway irritation. 



Sometimes peat moss, an organic, sterile product of ancient plant decay, is recommended for storing dahlias. Our experience has been mixed: peat moss turns tubers dark so may hide markings. It retains water like a sponge and thus is unsuitable for wet tuber storage. Yes, most sphagnum peat moss is inexpensive, but the environmental concerns over depletion of precious peat bogs have made peat moss - a non-renewable product that took centuries to form - anathema to committed gardeners. Kitty litter? (You have got to be kittin’!)  

Happy Holiday Season,

Martin Kral

Dahlia Talk Co-Authors Martin Kral and Dianne Reitan

at the 2023 ADS Show in Portland

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