Green Thumb: Growing Dahlias


Sam and I drove down to Van on Sunday  to hit the annual tuber sale hosted at the beautiful Van Dusen Gardens. The drive was nice and smooth and once past Squamish I scoped out plants obsessively from my window seat. Flowering currants growing cliffside,  followed by flashy azaleas and once in the city magnolias galore, cherry blossoms and daffodils.

We arrived at the sale half an hour late. I was picturing a zoo of dahlia-crazed ladies knocking  each other over for the last “tuber of the year” so I got Sam to drop me off while he hunted down parking. I ran in there like a serious nut! It was nice and mellow inside though and a great place to chat with experienced growers. We left with 100 new dahlias for our garden this season and lots of tips which I will share with you. I’m so incredibly pumped for the sea of colour these funny looking tubers will transform into!!

The best time to plant this group of flowers is in when your soil temperatures are at least 20 degrees – roughly around the same time you would plant tomatoes. Last year Sam and I planted ours out in the second week of May. You can lay down black plastic or landscape fabric to warm the soil if you want to get an early start. If you are really keen, go ahead and start some inside in pots but don’t water them too much. Just a light misting will do the trick.  The nursery in town has some nice dahlias in stock now – if you are looking to try them out this year, head on over and take a gander. We like the smaller-sized dahlia: they are much more prolific, which is good for us and they tend to flower sooner . We did snatch up some dinner-plate sized ones as well this year, so watch out! Don’t forget: if you want your dahlias to be as big as your face or a plate you will have to dis-bud.  (I will explain disbudding at the end of this post.)

We plant our tubers two feet apart and 3 inches below the soil surface in beds amended with composted manure and a handful of rock phosphate (which is available at the nursery in town). You could plant slightly deeper if you have sandier soil than us. Try to plant so the eye of the tuber is facing up, but don’t worry, the sprout will find its way otherwise. Sometimes when you buy tubers from the nursery, they come in mini clumps that are dividable. Each tuber only needs one eye and the size of the tuber does not determine the size of the final plant. To divide, find a sharp clean knife and slice, leaving about a half-inch of the mother plant in front of the eye. Let the wound heal over for a day before planting. 2641432-sprouting-dahlia-tuber

We let our dahlias grow to about a foot tall before watering. May is usually plenty moist enough to keep the tuber happy and the last thing you want is rot developing. The first watering should be deep and afterwards once a week is plenty, for us anyways. We are lucky to have soil very high in organic matter which retains moisture well. Start fertilizing your dahlias about 30 days after planting with a fertilizer low in nitrogen and again every three weeks after. Nitrogen is the first number listed on the fertilizer label. Too much of it will cause weak spindly growth.

Topping your dahlia is one of the most important steps in the road to a plant full of blooms. They are starting to sound like high maintenance plants, which they sort of are, but they are oh so worth it!!  I remember being quite nervous the first time topping our dahlias – I had this feeling like I was beheading the plant. It is only grooming though, not at all slaughtering! And you will be rewarded with a sturdier and more compact bush in the end. Topping or pinching is simply snapping off the central shoot of the plant.  You can use your fingers and the technique is quite simple. Gently grasp the tip of the plant and bend it back making sure you do not damage the two lateral branches to the left and right of the pinch. Depending on the size of your dahlia, topping is performed at different times.  I made a chart below that shows you what I mean.

Flower Size                     # of Pairs of Leaves

AA   (Dinnerplate)                      2 or 3

A      (Large)                                  3 or 4

B       (Medium)                                   5

BB     (Small)                                       6

If you are looking for larger, showier dahlias then the final thing you need to do is disbud. This is removing the smaller buds that surround the main bud at the tip of a growing lateral. The technique is important when trying to get those dinner plate sized blossoms. Disbudding works by channeling the plants energy into creating one larger flower. In some cases the tip of a lateral stem may hold 4 buds and all of those buds are trying to flower. The simplest way to disbud is to use your fingers and as soon as the buds are pea-sized as gently and softly as you can pinch out the smaller buds surrounding the largest one.


Try bringing some inside your house. They love to be cut and if you keep your plants healthy they will keep flowering right up until frost!!

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