Digging Time

Digging to Divide Dahlia Tubers

Saying good by to plants in the fall is hard, but when you dig a mass of tubers from the ground and see the energy of the plant placed into next years tubers, it seems ok to let go. In fact you may feel like you just one the lottery. There are a few steps to digging your dahlias and prepare them for storage. In the images, videos and blocks below, we will walk you though information to make this process as simple as possible and give you tips to help you successfully pull your plants either for storage, or immediate replanting if you live in a climate like ours where tubers can stay tucked in the ground year round.

When to pull plants from the ground is the first idea to consider. In some environments, you may wait years for a frost. In other locations, you may be desperate to pull plants before the ground freezes hard. In general the plants need a minimum of 120 days to really form tubers. The changing of temperatures in the fall appears to influence the plant into investing energy into the tubers. The number of tubers can be partially related to the number of days the plant is in the ground, plant genetics, and plant nutrition. In 2022, we had a late plant due to wet and cold spring, followed by an early frost, which resulted in small tuber clumps that were easy to lift and divide. We prefer the years where we have to wrestle tuber clumps from the ground. Plants don’t count, so the 120 days is an estimate and if you can push it to 130 or 140 days, the plants will have more time for their tubers. Try not to hurry this process. Swan Island recommends waiting until November 15th or a killing frost. This has worked for us.

If you do not get a frost, you can prepare the plant and encourage the tubers by cutting the plant back a bit. In years where we wait for a frost, but the storms have ravaged the plants, we go through and cut the plant back by half it’s height about a week before we want to pull it. We then go back through a day or two before we plan to dig and cut them to 6-8 inches. The first photos you will see below are from the process we just described. It is important to disinfect your tools between each plant to mitigate disease spread.

If you get a frost (see the photos of blackened plants towards the end of this tutorial), the plant will be shocked. You can go through and cut back all the plants, but the big open stalks may gather water, so we prefer to cut down the frosted plants just before we dig. Stalks that have frozen will get water inside even if you cover them because the cells break during the freeze and release their water into the plant. You can start to dig your plants anywhere from 1-2 weeks after the killing frost. Some climates allow gardeners to slowly dig all year as the tubers are insulated in the soil, but tubers in the ground in locations where the ground freezes will want to lift the plants before the freeze gets down to the layer of soil the tubers lie in. Even if the plant has died back, disinfecting tools between plants is a good practice to prevent disease spread in your garden.

Note in climates that are mild, after a frost if the temperatures raise then the plants may try and grow new plants immediately. We try and dig the whole garden in one week of the kill frost because our plants will try and regrow. The longer our tubers sit, the more likely an eye will become a small stem and we want that energy saved for the next spring. If you have plants that have been in the ground longer than a week, you may expect to see white, shoots lacking chlorophyll as the plant tries to replace the killed off stalk. This is not necessarily an indicator of disease. You can see clear and distinct margins around the shoots to the right. The shoots are different sizes. This is ok. This is a tuber trying to grow a new plant.

Start with cutting the stalk back to 6-8 inches from the soil, as you can see below. You may want to have a label on the plant if you like to keep track of your varieties. Now it’s time for digging dahlia tubers.

It doesn’t really matter what you use to lift your plants. Some people like a garden fork, others a thin shovel. We tend to use a shovel and start our hole at least 12-14 inches from the plant. Some dahlias make long skinny tubers, so we want to loosen the soil far enough away from the plant that when we insert our garden tool, it does not chop the tubers. If you are lifting a plant that has been in the ground for two years, you should consider starting 14-18 inches away from the stem.

After the soil is loosened all the way around the clump, the fine roots should have popped a bit and released from the soil. This is important because if these roots are still tightly imbedded in the soil, they will strain the necks of the tubers as you lift the stalk. The goal is to have the whole ball of tubers loosened.

Now, insert your shovel or fork back from the stem by the width you loosened the soil with. You will apply gentle leverage on the shovel to get the whole root ball to come up. We often apply leverage with one hand and use our other hand on the stalk of the plant to gently lift and support. If there is too much resistance, not where. Lower the root ball back into the ground and move your shovel to the area of resistance. Wiggle and loosen the dirt there, and try to leverage up the root ball again. This process helps reduce broken necks that happen when plants are pulled from the ground like a weed. This is why many commercial growers and imported tubers have broken necks.

Provided you have nice light soil and have not dug after a big rain, the soil should be easy to brush off the tubers at this point. The video to the left shows how we brush excess soil to remove the tubers. Maintaining a bit of a stalk on the plant helps provide a handle for gently manipulating the tubers.

In our dahlia patch, the process of digging dahlia tubers is repeated and we begin at one end of the row and continue down the line. The plants are cut back and plant material is put into a compost pile. You may have heard that dahlias are susceptible to some diseases (bacteria causes two galls and viruses impact plants too). We lift the tubers and leave them on top of where they were dug. We then remove them to the lawn where we can wash the soil off. We have had the same plants for years, so we move them to the lawn in groups for their hose down.

Be a steward of cross contamination. When we introduce new tubers, we move them and wash them one at a time, always in a new location on the lawn so as to not cross contaminate if we find gall. If you are new to tubers, you may also want to do this one at a time. If you discover a plant has gall, you will want to dig the soil 18 inches around the plant and 9-10 inches deep into a trash bag and toss it (or move it to a different area of your property where the contaminated soil won’t mix with material you will use in your garden (Do Not Compost it). The same goes for the plant. It is all contaminated and can affect not only dahlias, but peonies and many other plants too.

Make sure to get rocks, bark and any clay off the tubers. Clay can draw moisture from the tubers in storage causing them to wrinkle. It also hardens considerable and makes dividing extra challenging.

There are a few final notes before you divide. The tubers like a potato can dry out. Once you have washed the tubers, let them dry for a few hours. This is really so that you can label them with a sharpie if you want, and so that your hands don’t get cold and wet in the process. You can wait up to 24 hours, but after that, the tubers may start to dry out in your area. We dig in small batches so that we have time for dividing or at least getting them into storage. In many areas, even after you get the first frost or freeze of the year, the tubers nestled 4-6 inches in the ground are insulated and protected. This gives you a bit of time before they have to be dug. Once they are divided, they will need another 12-24 hours for the cuts to develop a skin before storing.

Even if you are confident that your plants are healthy and you do not have disease, its risky to compost the plants into the soil you plant to plant dahlias in the next year. Gall bacteria may be present, but inactive and can be added to the soil during composting. The American Dahlia Society Virus project in conjunction with Washington State University reported in the March 2023 update that 87% of the dahlias they sampled that looked healthy had a virus. Any plant material would act as a reservoir. This is most important with TSV, a virus that is spreadable by touch alone. I haul all the waste to a compost area outside of the dahlia patch so it can compost. There are many added benefits to this, including the plant material will provide a winter home to beneficial insects and bees, while returning nutrients to the soil.

If you are curious about gall, we are too. It’s a blessing we have not had these diseases in the over five years we have been growing dahlias. However, when keeping notes and building a website to help others enjoy these flowers, it would be handy to have pictures.