Volunteers Maintaining the Largest Public Dahlia Display Garden on the East Coast | Based at the Bayard Cutting Arboretum in Great River, NY
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Early Season Care: Tubers planted directly into the soil do not require watering in, but rooted cuttings should be watered regularly to prevent drying out. There is enough moisture in the spring soil for the tuber to root and grow. When the sprouts emerge from the soil, they are in danger of being eaten by slugs, snails or cutworms. A small collar (like the top half of a Styrofoam cup) into the soil and around the sprout will deter cutworms but slugs need to be killed because they can crawl over the collar. Slug bait should be administered to the soil during this time. Deadline, being the best product I have come across, but is not safe for pets and little children.
Preparation: Dahlias need to be staked and tied to prevent breaking of stems due to the heavy bloom load. Stakes can be heavy Bamboo, Tomato stakes, iron rebar, plastic coated steel stakes.
Where: 6 hours of full sun. Because dahlias tend to get bushy and send out long feeder roots, plant dahlias 30 inches spaced out in rows 36" apart. Keep in mind that the dahlia stem will be 2-3 inches away from the stake.
When: To prevent frost damage to the young shoot growth, plant potted tubers around Mid-May (after last frost). If you plant tubers with only eyes directly into the soil you can plant a week or two earlier, because the shoots will not emerge from the soil for that amount of time. Do not plant any earlier. Tubers will tend to rot in the cold wet spring soil and they will not produce any growth till the soil temperatures are up to at least 50 degrees.
If the tuber was started in a tray or pot and has leaves they must be hardened off by gradually exposing it to direct sunlight and cooler outside air temps a few days before planting. The same is true for rooted cuttings.
How Deep: Dahlias need to be deep enough to protect them from sun and drying out, but not too deep to be wet all the time (rot)
Sandy soil 4"-6"
Rich organic soil: 3"-4"
If fertilizer was not added to the entire dahlia bed before hand, then you must fertilize using the hole method. Dig the hole next to the stake about 2-3 inches deeper than where the tuber would be. Mix in about a handful of 5-10-5 or 5-10-10 into the bottom soil. Then place about an inch of soil above the fertilizer, so that the tuber’s fine roots will not come into direct contact with the fertilizer. Place tuber on top of soil buffer horizontally with the eye or shoot facing up and about 1-2 inches away from the stake. If planting rooted cuttings remove the root ball from the pot and tease the roots from the rootball. Plant the cutting about 2 inches away from the stake. Backfill appropriately. The leafed out tuber or rooted cutting will most likely wilt a little after planting. This is normal. Plant late in day when the forecast calls for a few cool and cloudy days. If the weather won’t cooperate, then protect the leaves from the mid day sun by propping a tray against the stake to the south of the tuber.
Before the actual task of digging your dahlias, there are a few things to do so you have an easier time next year when you take them out of winter storage. Go through the garden and ensure that all your varieties are marked with the correct variety, adding any pertinent information that could help you next year (too tall or short heights, favorites). Also mark extremely good stock, in case you want to take cuttings to multiply that variety. Throw away any plant that has a virus, is stunted from a virus or yielded inferior blooms. You have only a limited number of spots to plant your dahlias, so don’t plant anything questionable. You can always buy new stock. Be sure NOT to discard unwanted tubers and dahlia stalks into your compost pile. Send all dahlia refuse to the curb or to the landfill. Why chance spreading virus, spider mites or cyclamen mites to your compost.
Before you take out your shovel, transfer all the tags to the base of the plants. Look across all the stakes to make sure all tags have been moved. This will help prevent creating "no-name" tubers. It might seem to be a sin, but to me a tuber without a name is worthless.
The next question would be is "When to dig". According to a recent experiment reported in a handout by Kevin Larkin, dahlias produce tubers under shorter light conditions. Thus it would seem that the longer the tubers are in the ground at the end of the growing season, the more fully developed they will become. While most people wait until frost to dig, some can’t, due to the large number of plants they must dig. Small growers should seriously consider letting their dahlia roots continue to grow and mature as long as practical.
When you have decided when you will dig your tubers, cut the stalks about a few days to a week before digging. This will enable the tuber to start developing eyes in the ground and will help you later when you divide the clumps into individual tubers. Leave about 4 to 6 inches of stalk above the ground to make handling easier when you dig the tuber. If the stalk is open at the top, cover it with aluminum foil, so water cannot get in and promote rot.
Dig and handle the tuber clumps with care. A dahlia tuber’s neck is fragile, especially right after digging. The actual digging should be done with a long handle shovel. Do not use a garden fork. A fork won’t cut the fine roots free and much more damage will occur. Insert the shovel about 9 inches away from the stalk STRAIGHT DOWN into the soil and continue around the stalk in a circle. When all 4 sides are loose from the long feeder roots, push the shovel under the clump and lift carefully. If it is stuck, you will have to further cut the fine roots where it is stuck. Remove any large clumps of dirt and gingerly shake the tuber clump to further loosen the soil. Turn the tuber clump over to drain any water that has collected in the stem. At this point there are a few decisions to make.
Leave the dirt on: Let the clump dry in a cool area to prevent shriveling. The dirt will generally fall off with a little shaking. Move and secure the tag to the base of the stem and trim the stem and all long thin roots.
Wash the dirt off: Using a garden hose, wash the dirt off as much as possible. Move and secure the tag to the base of the stem and trim the stem and all long thin roots. At this point some people dip the tuber clumps in a 10% Clorox solution to kill any hidden worms, insects or microorganisms.
In either event, washing or not, the most important step in successful winter storage of dahlia tubers is the curing process. The outer skin of a dahlia tuber is somewhat damp when dug and downright sopping when washed. This outer skin must be slowly dried so as to toughen it enough so you can write on it with a felt pen without rubbing the skin off. At that point the tough skin will prevent further dehydration of the tuber and shriveling. Curing is accomplished by loosely covering the freshly dug or washed tubers with a plastic tarp. The tubers should be inside a cool space, like a garage, screened in porch or unheated basement. Do not place them on cement, since it tends to draw the water out of the tubers too fast. If outside is the only recourse, they should be kept out of any direct sunlight, as this will possibly cook the tubers. They should be controlled dried in this fashion for 4 to 7 days, checking them every day for dryness.
It is much easier to divide roots in the Fall, since the stem hardens over the Winter. In fact, it may take a set of loppers to cut through the stem in Spring. On the other hand, it may be harder to find the growing eyes in fall before they start to sprout. In either event, each division must have a piece of the crown with an eye. Remove all of the stem, because any piece remaining tends to promote crown rot and ruin the tuber. Also throw away the "Mother Root" (initial tuber planted in Spring), as it tends to rot and could spread to other tubers. Tubers need only large enough to keep through the winter without shriveling. Many experienced growers prefer small tubers to large tubers, unless they plan to use the roots to take cuttings(and then throw them away). Some varieties seem reluctant to develop feeder roots if their tubers are large. For these varieties, smaller tubers (or large tubers with all but the top inch or two cut off and thrown away) will produce stronger plants and better blooms than large tubers. If the inside of the crown has any brown or rusty colored divisions, cut them away. The discoloration probably indicates crown rot, and the tuber is unlikely to keep. After cutting divisions, use a hose or indoor wash tub to clean away any dirt missed when first washing the clump. Treat any cut end of a tuber with either sulfur, or a fungicide like Captan or Cleary’s 3336.
There are numerous methods to store tubers over the winter. Various methods seem to work equally well as long as it has the same characteristics of a root cellar environment. Root cellars are generally 40 to 50 degrees and have dirt floors which provide an exchange of moisture. The temperature requirement should be easily supplied by using an unheated basement, crawl space or the inside wall of an attached garage. But the moisture requirement is kind of a tricky thing to replicate. It’s important to keep in mind that any further moisture given off by the cured tubers must be absorbed and trapped in some kind of companion storage medium so the tubers will not continue to dry up and eventually shrivel.
Most growers seem to use vermiculite in plastic bags or Styrofoam coolers to store tubers that have been cured (controlled dried for 3-5 days). Course vermiculite works better than fine, due to being able to keep trapped moisture farther away from the tuber. Always use a dust mask when working with vermiculite. Perlite has been used, but most do not recommend it, because perlite does not absorb moisture. Experienced growers warn against peat moss, as it tends to draw moisture from the tuber and make it shrivel.
Many growers use newspaper as the storage companion for moisture retention. A sturdy cardboard box lined with about 12 sheets of newspaper on all sides, bottom, sides and top make an inexpensive method of storage.
The storage location does not only determine the temperature, but also the average humidity of the tubers. Again the temperature should be between 40 and 50 degrees, but you should judge how much dry winter air is being circulated through the general area of storage. An active garage where the door is opened daily, should not affect the average temperature, but it will dry out and shrivel tubers, even if they are in a closed cardboard box. The best thing to do is enclose the cardboard box in a leaf bag, so any moisture leached out of the box will be trapped in the plastic enclosure.
Dipping tubers in paraffin was fairly popular several years ago. Tubers dipped in wax tend to be very slow to develop eyes. The failure of this method to become popular probably indicates that the extra effort is not worth the trouble.
A recent method described in an ADS bulletin was the "Mandella method" using Saran wrap. The cured and divided tubers are wrapped in Saran wrap in such a way as they are not touching each other. The Saran wrap is pulled from the roll and is totally wrapped around the tuber, then subsequent tubers are wrapped in the same piece of Saran wrap. Only tubers of the same variety (properly marked) should be wrapped in the same bundle, making it easier to catalog what you have. Click here for more information on the "Saran Wrap" method. The wax and Saran wrap method would probably be much more forgiving of a dry storage environment as long as the tubers were properly cured.
With any storage method you should always check your tubers monthly for any signs of fungus, rot or shriveling. A spritz of Alcohol should keep any fungus in check. Discard any tubers that have rotted before it spreads to others in the container. If the tubers show any signs of shriveling dampen the tuber and medium slightly and enclose the container in plastic if not already done so.