The first question people ask about the taking of dahlia cuttings is "Why bother taking dahlia cuttings at all, when all I have to do is plant my tubers?" Actually there are several good reasons why taking cuttings might be advantageous to the average dahlia grower as well as the more experienced grower. The most important reason would be to quickly increase your number of plants from a single tuber. If you plant a single tuber you will only get one plant the first year and then more plants the second year by dividing your tuber clump at the end of the growing season. If, however, you force your single tuber to send up shoots in March and April, you can cut off those multiple shoots, place them in a rooting medium, pot them up when roots have developed, and have 5 to 10 plants to plant out into your garden about the middle of May. When you spend $15 or $20 for those fancy new releases and you get 10 new plants instead of only one, then the cost basis of your plants becomes considerably less. Plus you can share the extras with your friends and neighbors.
A second reason for taking cuttings is that it gives the growing season a head start. One foot high plants set out in the garden in mid May will bloom sooner than the tubers that you planted around the first of May (Average date for planting dahlia tubers in the Pacific Northwest). Therefore, you will get to enjoy your dahlia garden for a longer time.
A third reason for taking dahlia cuttings is to save that hard to grow dahlia. Some varieties (Camano Cloud) don't always grow very well from tubers but do fine when grown from cuttings.
If we have convinced you to try taking a few cuttings this year, then the next question would be “When do I start?” The answer, depending on your resources and depth of involvement, is some time between the first of February and the first of March. Start earlier if you plan on taking a lot of cuttings; later if you’re only doing a few.
People often ask, “ Don’t I need a greenhouse to take cuttings?” The answer of course is that it would be nice, but certainly is not necessary. If you have a greenhouse by all means use it, but if you don't, use your basement, garage, spare room, small space on the kitchen counter, or a window sill. The key elements are light, heat, and moisture. If you can introduce these either naturally or artificially then you will have a place to grow your cuttings.
Sometime in late January or early February gather those new and hard to grow tubers and pot roots
(Cuttings from the previous year grown in 4 inch pots) from which you are planning to take cuttings.
Place them in individual pots if you’re only taking cuttings from a few tubers. When taking cuttings
of numerous tubers, then I recommend using six inch deep plastic trays (16” x 22” or 10” x 16”,
available at most large chain hardware stores. Fill pots or trays to within one inch of the top with
lightly moistened potting soil or a half sand and half peat moss mix. Plant your tubers vertically in
rows spaced about 4 to 6 inches apart being sure to place a variety label next to each tuber. Expose
the neck of each tuber an inch or two above the soil level to make the taking of cuttings easier.
Finally, place trays or pots about 6 to 8 inches below florescent shop lights. No bottom heat is
needed if set up is in a heated room (60 to 70 degrees). If set up is in a cool garage or basement
then bottom heat is recommended. Sources of bottom heat could be heating cables (cheap, $25),
special heating mats (expensive, $100+), or the top of the water heater or freezer (free). Most tubers
will begin to eye up in a week to 10 days. Some, however, may take up to a month while others
spring forth in 3 or 4 days It depends upon the variety.
When shoots have developed two or three sets of leaves and are about 3 inches tall, it is time to take
your first cutting. Using a one sided industrial razor blade or a sharp hobby knife, slice off the dahlia
shoot about 1/16 inch or the width of a dime from the crown of the tuber. Four new shoots will now
develop from the base of the first shoot. Keep taking cuttings until you have all you need or until the
cuttings become thin and weak. Warning: No new shoots will develop if you slice off the base of the
shoot where it connects to the tuber crown. Pot clumps are great for taking cuttings because they eye
up quicker than single tubers and they also send up more shoots. Always grow a few of your favorite
varieties in 4 inch pots so you will have a good source for cuttings the following year.
Remove the bottom set of leaves from each cutting and place the stem in a small jar or pill bottle of
fresh water for several hours before sticking into a rooting medium This allows the cutting to take up
as much water as possible. Don’t forget to label a tag with the variety name and date taken. Put the
tag into the jar or bottle along with the cutting.
Finally, to prevent the spread of dahlia virus, wash your cutting blade between each cut in a dish detergent and water solution. We’ve been informed by the dahlia virus research team at Washington State University that this method is all that is necessary to kill any dahlia virus.
There are two popular rooting mediums to use. The most common is to stick cuttings directly into a four
inch pot filled with a high quality potting soil mix. The potting soil should be completely saturated with
water before planting and must be kept very moist until the cutting has rooted. Most people dip the cut
end into a powder or liquid rooting hormone prior to sticking. Insert the cutting into a half inch deep hole
made with a dibble or pencil and press soil firmly around the stem to remove air pockets. Place over
bottom heat and under florescent lighting until rooted. Move cuttings outside into the cold frame in March
and April and harden off before planting in mid to late May. The advantage of this method is that it’s a
one step process. The disadvantages are that it’s messy indoors and that your loss due to damping off
(cutting wilts and rots) can be high depending on your conditions.
A new system gaining popularity in the Pacific Northwest is the use of “Oasis” wedges as the planting
medium. These wedges (wedge shaped “Oasis” foam, 1” by 1” by 2”) are saturated with water and
placed in special trays that self water from below. The top of the wedge has a precut star shaped hole
that allows cuttings to be inserted approximately one half inch into the wedge. In a study the author did,
it was determined that cuttings actually root a little quicker without using a rooting hormone. A mild
fertilizer is incorporated in the wedge system by its maker. For best results only use every other slot in
the special tray and don't pre-soak the wedges until they are needed. If your local nursery does not
carry the “Oasis Wedge System” then ask them about ordering it. Greenhouse and nursery supply
businesses will either have it or will be able to order it for you. Its advantages are numerous. There is
very little mess in the garage, basement or spare room. The cuttings root quicker (usually 5 to 12 days
depending upon variety). There is absolutely no damping off. Occasionally a cutting will fail to root, but
we’re talking about 2%. The disadvantage is the extra step. After rooting the cuttings still need to be
potted up into 4” pots and placed under florescent lighting (14 hours per day) until they can be moved
into the cold frame. I store mine in the garage on steel industrial shelving purchased at COSTCO. The
shelves are four feet long by 18” wide and allow two shop lights to be hung under each shelf, lighting
the cuttings beneath them. The nice thing about storing cuttings this way is that they grow low and
compact in the cool garage environment. Just make sure they don't freeze!
Some final observations: Remember, that what ever works for you keep on doing it, but don't be afraid to experiment. I have lost tubers to shriveling and rotting after I have benched them up. There are no guarantees that all your tubers will survive. I now put tubers into a plastic bag along with slightly moistened potting soil about a week before benching to help them eye up and to get their feeder roots started. I don't use plastic domes over my cuttings as I believe too much heat and moisture promotes damping off. I will fertilize the cutting with a half strength all-purpose liquid fertilizer about once a month until they are planted into the garden. I add one scoop of 14-14-14 Osmocote to each hole at planting time. You can take a few cuttings without investing a lot of money in equipment. Just adhere to the dahlia cutting’s need for light, heat, and moisture. Your cuttings might be gangly and ugly, but once they are topped, by mid-summer no one can tell the difference.