Winter Storage of Dahlia Tubers

By Jonathan Wooten

Nearly every experienced grower of Dahlias has a slightly different method of storing tubers over the winter.  Part of the reason is that each grower has different options and spaces available to store the dahlias. I have found that that two factors are critical for successful winter storage of tubers, consistent cold but not freezing temperatures and humidity. After years of spotty success storing tubers in a house without a suitably cold space, I devised a method which has a 98% + survival rate which is in my detached unheated garage. For those who don't have that convenient cold frost free spot, this could be a solution.

This method begins with an insulated box with a lid which is sized to hold 6 plastic garbage cans. This box is framed with 2x4's, sheathed with 1/2' plywood and insulated with 3 1/2 " fiberglass insulation. I added 1/2" of urethane foil faced board on the inside protected with thin plywood over it, but this may be over kill for all but the coldest of climates. The size depends on how many roots are to be stored and the dimensions of the root containers. This particular box has a structural framed top which is also insulated and hinged to allow ease of opening. The pink material is foam sill sealer to prevent air infiltration. When the top is closed, it can be used as a bench or a counter in both winter and summer. If a structural top to the bin is not needed, several sheets of insulating board could be used. The total cost of materials including the thermostat is less than $200.
A greenhouse thermostat was installed on an adjacent wall. This is a $75 Durostat unit with 1 degree accuracy and a 12' remote thermostat which is threaded through the lid, with the sensor next to the Dahlia tubers. The temperature set point is programmable, as well as the number of degrees the temperature can drop before the heater is switched on. I use 40 degrees, with 3 degrees drop to 37 degrees when the heater is switched on. Since a 40 degree reading instantly shuts off the bulbs, and 37 degrees instantly turns them on, the temperature readout is either 38 or 39 degrees for 98% of the winter. The uniformity of the temperature reduces the heat/cool condensation cycles which I believe contribute to rot. The output line goes to two 100 watt light bulbs, the heaters. Two bulbs are used merely for redundancy. I have found that in Connecticut, the light bulbs never go on in December, and the duty cycle in January and February is probably less than 5% even during a brutal winter. The power cost is less than $5 for the entire winter.
Preparation of Dahlia tubers is similar to the approach used by most Connecticut Dahlia Society Members. I put dry to the touch, divided Dahlia tubers of a single variety in plastic shopping bags which are filled 3/4 with Vermiculite. Vermiculite absorbs water which is given off by the tubers over the winter and keeps the air humid. The bags are left partly open and are placed in 2 gallon plastic pots only for support. This allows good air circulation in the garbage cans since they are filled to the top. One can will hold 11 pots. One pot can hold up to 40 roots so more than one bag can be placed in a pot if there are fewer roots. In total, the bin can hold over 2,000 roots.
The plastic garbage cans are covered and placed in the bin for the winter. Since the environment in the bin is cold and  extremely stable at 38 - 39 degrees, any tendency to rot is greatly retarded. I do not check the tubers over the winter, but glance at the illuminated thermostat occasionally when in the area. When the spring comes, the tubers are in excellent condition looking little different from when they went in in the fall. I have found that the large thermal mass of the roots in the bin and the concrete floor below combined with the protection of the insulation provide a high level of frost protection in the event of a power outage of less than a couple days.

Since the cost of the storage bin is nearly the same regardless of its size, growers could consider sharing a unit. This configuration holds 6 garbage cans, and each can could hold about 400 roots. More could be stored if the containers were rectangular bins.

The only component not cheaply available at any big box home store is the thermostat. This Durostat thermostat  may not be still available, but a similar and possibly superior Ranco unit is available for the same price here. The Ranco remote thermostat cord can be much longer than the Durostat's 12'. There are even cheaper thermostat units available, but the remote temperature sensor feature which allows viewing the interior temperature without having to open the bin is very convenient and is well worth the money.

There are a number of other approaches to storing tubers over the winter. Some people use a crawl space or an unheated basement and others use an insulated attic. One friend uses an entryway to the basement under a Bilco door, and has fitted rigid insulation under the surface under the door. The residual heat in the ground is enough to keep the temperatures above freezing. Attaining the ideal is hard to achieve, i.e. complete protection from frost yet maintaining consistently cold temperatures. In most spaces colder temperatures increase the risk of frost on extremely cold nights. The thermostat controlled bin is a good, safe, economical way to store Dahlia tubers over the winter.