Bursting into bloom in the heat of the summer when many plants have finished their seasonal show, dahlias churn out gorgeous blooms non-stop until the first frost of autumn. Anyone with a sunny spot should have at least one of these spectacular plants in their garden. Read on for everything you need to know to grow great dahlias.
BASIC CULTIVATION
Dahlias are very easy to grow. They need full sun - no less than six hours, preferably more - and regular water. They are big, fast growing plants, which means they use a lot of energy. After planting, consider sprinkling a handful of bone meal on top of the soil, around the outer edge of the rootball, along with a balanced fertilizer (10-10-10 or thereabouts) applied according to the package directions. If you want your dahlias to really shine, consider applying a fertilizer formulated to promote blooming once you see the first tiny flower buds forming. A bloom-promoting fertilizer is one in which the first and last numbers should be below 10, and the middle number should be above 10 (Eg. 4-14-6). If you do not fertilize your dahlias, you will still have lovely plants; they will just be less vigorous than they could be. If you’d like to learn more about fertilizer formulations, subscribe below to be notified of an upcoming article on the topic.
PINCHING AND SNIPPING
In order to promote nice bushy growth, we recommend that you “pinch” your dahlias. We take care of this for you when you buy your initial plant, but it is good to know for future seasons with this self-multiplying perennial. Pinching is completely optional, but gives the plant a nice shape. It is very simple: when the plant is between 12 and 16 inches tall and has four or five “branches” of leaves on the central stem, look for the central bit of growth between the top two leaves and pinch or snip it off. The plant will interpret this as a message to grow more lateral (sideways) branches, resulting in a fuller, bushier plant.
In terms of flowers, there are two camps of dahlia growers: those who want lots of flowers, and those who want larger flowers. If you would like a riot of flowers no action is necessary; your dahlia will bloom happily as long as it is adequately sited, fed and watered. If you would like larger flowers, some very easy pruning is required: on each branch with buds, you will usually see three buds forming – a central one and two side buds. Pinch or snip off the two side buds. This will divert all of the energy that would have been used for those two buds into making the remaining bloom larger. It will also result in a longer stem for the remaining flower, which is useful if you intend to harvest the blooms for floral work or exhibition.
Pinching and snipping are just two tricks to make a great plant even better. Neither is required in order to have a lovely, floriferous plant.
HARVESTING AND VASE LIFE
Dahlias are not a terrifically long-lasting cut flower. They will last up to a week in a vase. To ensure you get the longest vase life, harvest dahlias when the blooms are just about totally open, and the green base of the flower (where it attaches to the stem) is firm. Immediately after cutting the stem, place it in a bucket or jar of water. It may sound like overkill to bring a jar of water outside with you to cut a few stems, but it makes a big difference. Take cut blooms inside promptly (they will wilt if left in the hot sun). Fill a high-sided container (a vase or a jar) with about 10 centimeters of the hottest water you can get from your tap. Put the cut stems into the container and leave them on the counter for at least two hours before either adding more water and enjoying as-is, or using the stems in a further arrangement.
LIFTING DAHLIAS
Dahlia tubers do not like to be left in the ground over winter. At the end of September, the roots of the dahlia (long yam-type things called “tubers”) should be dug up and stored in a dry, cool place such as a garage or basement. To dig up dahlias, use a shovel or digging fork, and put it into the ground at least 20 inches away from the base of the plant. Any closer and you are practically guaranteed to slice a few tubers in half. Loosen the soil all around the plant and then, using the shovel as a lever and rocking the stem gently back and forth, ease the clump of tubers out of the soil. If you are digging up more than one type of dahlia at once make sure to have separate areas to pile each type as you dig them up – tubers are hard to tell apart.
Rinse the tubers off with the hose and let them dry. (If you feel very strongly about your dahlias, the best practice is to swish them about in a mild bleach solution to kill any disease or fungus that may be lurking in the soil. (We do this for our nursery stock, but find that it is not necessary for most casual home growers.) You may, if you wish, write the name of the dahlia on the dry tuber with a permanent marker. Once the tubers are dry (and not too long after that, or they will start to shrivel) put them in a labelled cardboard box or paper bag, with at least an inch of vermiculite underneath and on top of the tubers. This will keep them from drying out over the winter. Close the box or fold over the top of the bag. Do not seal with tape - you want a bit of airflow.
(Note that if you search "storing dahlia tubers" on google, you would find a boatload of advice which differs substantially from what you have just read. This is because the manner in which dahlias are stored changes a great deal based on climate, especially humidity. Growers in Arizona may do just fine sealing their tubers in ziplock bags, but if you were to do that here you would have nothing but a bag of mold to plant in the spring.)
When you take out the clumps in the spring (either in April if you want to plant them in the ground, or in early March if you want to start them off in pots and get a jump on the growing season) they will need to be divided before they are planted. An in-depth article on dividing dahlias is in the works - subscribe to our newsletter at the bottom of this page to be notified.
What happens if you don’t lift your dahlias? One of several things: If they are in soil that does not drain freely, or if we have a very wet winter, they may rot. If they are not planted at least 8 inches deep and we have a cold winter, they may freeze and subsequently rot. Or you may get lucky, and find that some of the tubers survive. Lifting is undeniably annoying. For this reason, some people on Southern Vancouver Island treat dahlias as annuals and leave them in the ground – if they come back, great, but don’t count on it. The reason we go to the trouble of lifting our dahlias every year (and honestly, it’s not that bad once you’ve got a system) is because that dahlias will provide a never-ending supply of new tubers if you're kind to them, each of which will grow its own clump. Your dahlia stock will multiply exponentially each year. Once you have stuffed your garden full of fabulous dahlias, you will have tubers to trade or to pot up and keep ready for use as hostess gifts, end-of-year teacher presents, et cetera. Give a classy dahlia as a gift and prepare to go down in etiquette history.
PESTS AND DISEASE
Dahlias are not terribly prone to pests or disease, but there are a couple of things to look out for. On the pest front, the only real nuisance is earwigs. Apart form being just about the grossest creature known to man, they also like to nibble on dahlia petals. This makes them look ratty and also means that - HORROR OF HORRORS - there is sometimes an earwig or two in your cut flowers. Which are in your house. Earwigs in your house. To avoid this revolting situation, turn your dahlia blooms upside down before you bring them in, and give them a few light raps on the side of the house (not hard enough to damage the blooms, of course). Alternately, you can submerge the flowers of the plant entirely in a bucket of water for about 3 minutes (just turn the whole thing upside down and dunk it). The earwigs will wriggle free from the flowers to escape from drowning (and they will succeed and probably outlive us all, because they are practically un-killable).
Both of the anti-earwig strategies above are apt to shorten the vase life of your blooms by a day or so. If you want maximum vase life and no earwigs, a little bit of extra work is required. Serious dahlia growers by little organza gift bags and tie them over the dahlia buds before they open. This allows sunlight and air to flow freely, but not bugs. I know of one grower who swears that smearing an inch of vaseline around the stems below the buds will keep the earwigs off of them.
Thrips, pictured on the right, are little flying bugs that look like tiny pale-coloured slivers. They are not unique to dahlias. Most growers notice that they have a thrip problem because the leaves of their dahlias have little light dots and lines on them. Thrips rarely cause a serious problem to dahlias, especially if they are at the back of the bed where the leaves are not very visible. But a bad thrip infestation can be a problem to other plants, most notable gladiolus and roses. Thrips can either be manually removed by placing a drop cloth under your plant and shaking the bejesus out of it, or treated with a very effective organic spray called Spinosad (most commonly sold under the Monterey brand name). The only time thrips really must be completely eradicated is in the event that one of your dahlias is infected with a virus. Thrips spread dahlia viruses, most of which are fatal. If you suspect a virus and thrips are present then man the torpedoes, it's time for war.
There is little that bothers dahlias in terms of disease, but when a problem does arise, it is important to know how to identify it. The most common dahlia viruses are the mosaic viruses. They are characterized most obviously by yellowing of the veins in the leaves. Blooms will be crummy, the plant will be stunted and there will be a lot of horizontal growth. There is no treatment for the mosaic viruses. Plants must be dug up immediately and put in the garbage (do not compost).
Another relatively common disease is called leafy crown gall (I say "relatively" because neither of these afflictions are really all that common). Leafy crown gall is characterized by a whole bunch of skinny, sickly-looking shoots growing up from the tubers (roots) of the plant. It is a very rare disease so long as you are buying your plants from reputable, local growers. (Throwing shade your way, Home Depot.) As with mosaic virus, tubers with leafy crown gall must be immediately dug up and thrown in the garbage (do not compost). Often leafy crown gall is not discovered until tubers are unearthed for storage at the end of the season. If this is the case, you will want to prepare a disinfecting solution for your soil in order to prevent the disease from spreading. Please contact us for further advice on this.
If you're not sure about what's bugging your plant, please get in touch and we will do our best to help you identify the problem. Please do not ever bring any bug or suspect plant matter to us in person unless it is sealed in a ziplock bag.
If you have any lingering questions about growing dahlias, please send us an email or pay us a visit - we are always happy to talk plants.
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