Starting Dahlia Tubers

by Tony Evangelista

Dahlia tubers are thickenings of roots. We dig our plants in the fall and divide clumps of those thickened roots into pieces for winter storage.  In the spring we plant those with an eye or germinal center to start a new plant. One can simply drop that tuber in the garden when the soil has warmed to 55-60 degrees F and hope for the best. It is successful more often than not for most growers. I have done just that occasionally over the years when I was just too busy in the spring to start them earlier indoors. My success rate varied considerably depending on the moisture content of the soil at planting and with the subsequent amount and frequency of spring rain. Wetter soil at planting and soil remaining wet after planting contributed to greater losses. I could lose over a third of the tubers planted in years with wet soil conditions.

To minimize the above risk with direct planting many growers opt to start tubers in pots or trays instead of dropping tubers directly into garden soil. This allows one to get earlier blooms as a bonus. It also ensures greater success if done properly.

So what are the factors to consider when starting your tubers early?

1. Temperature. The planting medium should be warm—at least 55-60 degrees F. Warmer soil increases metabolic activity and promotes root formation and sprouting. Some growers have used bottom heat successfully in cooler locations like greenhouses or basements. I think one must be careful not to get soil too warm though. I personally wouldn’t go much above room temperature (around 70 degrees).

2. Potting medium. I recommend a potting medium that drains extremely well.  Usually that means one higher in bark content. The bark content doesn’t absorb as much moisture and the larger particle size of the bark promotes excellent drainage. I use a commercial bag mix of at least 50% bark. This mix provides a higher margin of safety with watering. I use a 52% bark mixture manufactured by SunGro Horticulture. It is sold under the brand names SunGro 52, Fafard 52 or Metro-Mix 852.

3. Moisture. Try to maintain the moisture of the potting medium at approximately the same degree that was present in the bag. That means no watering after potting tubers and only watering very lightly to maintain that level of moisture. Tubers have no roots to take up water. They absorb water directly. The objective is to keep them moist enough to prevent shriveling but not so wet as to encourage rotting. That concept is critical. That means watering very lightly or even misting depending on your own environmental conditions. Use your finger or a moisture meter to monitor the moisture.

I tray and pot much of my stock in the spring about a month before I intend to plant outdoors. I put the stock on 4x8 sheets of plywood that I’ve equipped with wheels so I can move stock out during the warm days and back into my garage during colder nights. I don’t allow rain to water  the tubers. I control the moisture by hand watering sparingly. The soil temperature does vary widely with this method, but it works very well for me.

I have actually started covering some of my pots and trays with plastic wrap after potting the tubers.  If I pot a single tuber, I use my own potting mix. My potting mix has about the same degree of moisture as the bagged commercial mixes, which I feel is just about ideal. I don’t water the pot or trays. I simply cover with kitchen plastic wrap and don’t uncover them until I see growing green shoots (Figure 1). Only then do I start watering or misting very lightly. The appearance of the growing green shoots usually corresponds to hair root formation by the tuber and hence the ability of the plant to take up moisture.
The potted single tubers are allowed to grow in their pots until they are hardened off and planted in the garden.  The trayed tubers are hardened off in their flats and planted in the garden.

The plastic wrap should be removed once tubers start sprouting. Then carefully begin hand watering as needed even though all the tubers in the tray may not have sprouted.  I don’t put my pots or trays covered with plastic wrap in sun until after they spout and the wrap is removed. The sun will heat the soil when covered with the plastic wrap. This solarization technique, which is often used to kill superficial weed seeds by some organic farmers, is not desirable in this case. The heating can raise the soil temperature to undesirable levels and destroy the soil’s micro environment. You may want to try this technique on a small amount of your stock and see how it works for you. I find it particularly useful when potting a single valuable tuber such as a new introduction that I just don’t want to lose.

I would like to share a technique I use on blind tubers (those without visible eyes) that Buddy Dean of Hilltop Gardens shared with me years ago. Put the tubers in a one gallon ziplock bag with the 50% plus bark mixture and zip the bag closed (above image on the left).  Then put the bag in a warm room of your house.  Check the tubers every few days for eyes and then pot or tray the successful ones up. Remember to not add additional water to the bark mix. Always keep your bags of potting mix tightly closed to prevent the contents from drying out.

In conclusion, your ability and skill to control the degree of moisture around your tubers in the spring will determine your success when starting tubers.