Why Dahlia Tubers Don’t Compost Easily
Like many dahlia growers, I end up with large quantities of non-diseased but non-viable tubers—first after planting time, and again after end-of-season dividing. Although dahlia leaves and stems are easy to compost, tubers stubbornly resist decay and often sprout in the warmth of my worm-rich compost bins.
What Makes Vermicompost So Powerful
Vermicompost, or worm compost, is a nutrient-rich soil amendment created through the breakdown of organic matter by earthworms. It improves plant growth by enhancing nutrient availability—especially nitrogen, phosphorus, and potassium—in forms that are readily absorbed by plants.
Vermicompost also boosts soil microbial activity, promoting beneficial bacteria and fungi that aid in nutrient cycling and disease resistance. Its fine texture improves soil structure, increasing water retention and aeration.
In addition, it contains plant growth hormones (like auxins and cytokinins) and humic substances that stimulate root development and overall plant vigor. Repeated studies have shown that plants grown with vermicompost often exhibit increased yields, improved resistance to pests and pathogens, and greater tolerance to environmental stress.
The Microbe-Worm Connection
Before I started gardening, I thought worms just “eat” vegetable matter. But worms like Eisenia fetida (red wigglers) do not have the enzymes to break down fresh plant material directly—at least not the way our digestive systems break down cooked food. Instead, they rely on microbes—bacteria and fungi—that colonize decomposing organic matter.
Here’s how it works:
Microbes colonize the leaves, food scraps, and other organic waste as it begins to rot.
This microbial action softens and partially breaks down the materials.
Worms then consume a mixture of the decaying matter and the rich microbial community growing on it.
Technically, worms are eating microbially pre-digested organic matter—along with the microbes themselves, which are a significant source of protein and nutrients.
This microbe-worm synergy is what gives vermicompost its extraordinary biological richness: you’re not just getting decomposed food waste—you’re getting a highly active blend of worm-cast microbial ecosystems and plant-ready nutrients.
That got me thinking: how could I take advantage of the biomass that hundreds of dahlias produce every season, while recycling those nutrients through the magic of worms? The tubers take forever to rot, and the worms can’t work their magic until they do.
After pondering that question, I came up with the idea of intentionally freezing my non-viable (and otherwise unwanted) dahlia tubers. I fill a kitchen garbage bag and stick it in the chest freezer for a couple of days. After they are solidly frozen, I empty the tubers into a garbage can next to the compost bins so they can thaw, soften, and begin turning to mush.
As I layer in “browns” like wood shavings and cardboard, together with “greens” such as grass clippings and kitchen waste, I place a generous scoop of mushy tubers right in the middle. Within days, the worms find them and go wild—gobbling up the feast and making babies to continue their good work.
We all know never to let our dahlia tubers freeze if we’re trying to store them. But I thought you guys might be interested to hear how (and why) I’ve started freezing some of mine on purpose!
