Marg and Geoff Allison knew nothing about dahlias four years ago. Now, with their retirement, they spend up to 30 hours a week taking care of them.
In fact, they are fairly in control of the dahlia borders at the Christchurch Botanical Gardens – and what many consider to be the best public collection of New Zealand-grown dahlias in the country, and, in all likelihood, the world.
Geoff and Marg decided to volunteer at the gardens in their retirement. They are addressed to the Friends of the Botanical Gardens and assigned to Section D, the prosaic name for the area that includes the formal rose garden and the dahlia border to its perimeter.
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Do you have the dahlia bug? Do not blame yourself!
“After he was shown how to lift and divide, the curator quickly let us go,” Geoff recalls. “We did not know anything about dahlias … We decided we had to learn better. We went online and discovered Dr Keith Hammett. ”
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From that day on, the two say, the world-renowned Auckland-based Kiwi dahlia grower was a “tower of power and a goldmine of information”.
When they joined the Canterbury Dahlia Circle in 2019, the couple proposed to judges, breeders and advisers, all assisting in sorting out the collection. “Some dahlias were not named, others were not named correctly or were not true to form, so work out what was what was the first exercise.”
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Plant breeder Dr. Keith Hammett was a goldmine of information, the Allisons says.
Over the past two seasons, many of the dahlias have been rearranged to deliver a more balanced performance, Marg says. “It remains a work in progress as we continue to learn the growth habits of each cultivar. In the coming season we have already identified 46 cultivars that need to be moved. ”
The plan is to lift and divide each plant every two years, aided by the database set up by the department’s former curator, Susan Sanders. The database, which is kept up to date by Marg, keeps track of the plants, their accession, position, deadline for removal and so on.
New cultivars are regularly donated by breeders, with Geoff also searching catalogs to keep the collection up to date and to ensure that the range of dahlias represents its many colors, shapes and sizes.
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Ashburton planter Alan Trott reviews dahlias for flower power and summer drama – “they are a must in any summer border,” he says.
A new nutrition program was devised after one of the national judges once looked at the collection and said it was starving, Geoff recalls. “After consultation with the curator, it was agreed that we could use Garden Landscape, a commercial product. We apply it when we lift and divide in the spring and again in January as a side cover. A light fertilization of blood and bone is applied in winter. Potassium sulfate is applied during planting and with a supplement in January. “Since we fertilized, the difference in plant power and depth of colors has been incredible,” he says.
Sorting out the irrigation gave another leap forward. “Since the drip lines were installed, we no longer make noise with up to six snakes dribbling to prevent water from getting on the leaves. We make deep water at least once a week by leaving the drip lines overnight. ”
Botanical Gardens policy prevents the use of pesticides, so another method had to be found to reduce the threat of one of dahlias’ biggest enemies, plumes.
“When you cut off the old plants in the fall, tabs in the dead reeds can overwinter,” says Geoff. “In desperation, we use a pyrethrum-based fly spray in the old vines just as new shoots emerge in the spring.”
Rolled up newspapers, aluminum foil or egg cartons are also good, Marg adds. “Place them in the bush, remove them in the morning and shake all the tabs out in a bucket of water.”
The dahlias are not covered. “Pea straw will be a good attraction for the gardens’ game, especially ducks, and without it we will not be bothered by snails and slugs.” Bark and other commercially available coatings are also avoided because they have seen weeds being introduced into them, and because of the possibility that they may contain harmful residual chemicals.
The couple also overcame another problem, powdery mildew. “We remove the lower leaves and space the plants to encourage air movement,” says Geoff.
In November, the border’s dahlias kick off the flowering season, which lasts until well into April, when it usually begins to cut. Although all dahlias like water on their feet and sunshine, some flowers can bleach in the sun fairly quickly. It is moved to less sunny places.
Perhaps proof of the Garden Management’s belief in the hard-working duo is the planned expansion of the dahlia beds, which now contain 184 New Zealand cultivars and 12 species from their native Mexico.
More New Zealand cultivars will be introduced and more space will be given to the tree dahlias, some of which can grow more than 6 m tall. All the more reason to make a note in your diary to visit the beds next season.