WHEN I FIRST started gardening, I thought dahlia growers were a bit odd. At first blush, dahlia growing seems full of unintelligible rituals. I could not quite understand how it worked or how it was possible that such absurdly large flowers could even exist.
In the fall, I would see gardeners hurriedly caching tubers in boxes of sand, bundles of tall stakes crammed into the garden shed. In the spring, huge swaths of the garden were left empty for months until, out of nowhere, sprouted 6-foot-tall plants with dinner-plate-sized blossoms. Of course, once you dig into the mystery a bit, you’ll realize how fun and easy dahlia growing can be.
Dahlias are fascinating plants. Originally from Mexico, they were first cultivated by the Aztecs and other indigenous cultures as an edible tuber. The plant made its way across the Atlantic as a potential new food crop, but Europeans were lukewarm on its culinary attributes. Instead, they fell in love with its showy flowers. What followed was a 300-year period of intense breeding and hybridization. These experiments led to thousands of cultivars in every size and color imaginable (except, of course, blue). Dahlia catalogs are like Jelly Belly stores: If you can imagine a variety, you’ll be able to find it.
The insane diversity of bloom size and color has been made possible, in part, due to the plant’s genetics. While most animals have two sets of chromosomes, many plants have additional pairs, in a condition known as polyploidy. While some species are triploid or tetraploid, Dahlias have eight sets, making them octoploids. All these additional chromosome pairs means there are many more combinations available, thus encouraging the process of speciation.
Given the grandiose nature of the dahlia bloom, it’s logical to think that these garden gems would be difficult to grow. However, if you follow the guidelines below, you, too, can have an octoploidal backyard wonderland next summer:
10 Tips for Growing Successful Dahlias
1. Plant them in slightly acidic, well-drained soil with full sun exposure.
2. Plant new tubers in the spring at the same time as your tomato transplants. Tubers should be planted horizontally, about 6 inches deep and spaced 2 feet apart.
3. Use an organic slug bait such as iron phosphate at planting. Slugs and snails love new, emerging dahlia sprouts.
4. Do not add fertilizer at planting time. In fact, I never add fertilizer to my dahlias. However, many growers recommend an application of low-nitrogen fertilizer once a month, through the growing season. You can start fertilizing once you see the sprouts poke out of the soil in late spring. It is imperative that the fertilizer has low or zero nitrogen, as excess nitrogen can lead to weak stems, small or blown blossoms, and unhealthy tubers.
5. Add a stake for each plant. While there are dwarf dahlias that might not require staking, most popular cultivars are 3 to 6 feet tall (or taller). Dahlias have hollow stems, grow quickly and carry a lot of weight. Adding a stake at planting time is the easiest way to ensure healthy, upright plants. It also helps you avoid accidentally stabbing your growing tuber later in the season.
6. Do not add water when planting your new tubers. Soil in the Pacific Northwest virtually always has adequate moisture in spring for the establishment of tubers. Watering new tubers is likely to lead to rot and kill the plants before they have even emerged.
7. Add lots of water once the sprouts have emerged. Growing dahlias require regular watering throughout the summer to produce their blooms. I recommend watering two or three times per week June through August.
8. Cut blooms regularly through the summer. Like any ornamental bloomer, the length of your season and total number of blossoms can be increased by regular cutting and deadheading.
9. Keep a vigilant eye for pests and diseases. The soft, waterlogged tissues of these plants are prime targets for insects and fungi. Key players include slugs, spider mites, earwigs, powdery mildew and botrytis.
10. Leave your tubers in the ground over the winter. Traditional dahlia growing requires that the plants are dug up each fall, and tubers stored inside through the winter. However, given our mild winters, I have found that dahlias perform much better when left in place through the winter. Instead of digging up your plants in the fall, simply cut down the stems, add a few inches of mulch over the crown and wait until spring. In April or May, you can dig up, divide and replant your tubers as soon as you see the new sprouts emerge. It’s best to let the divided tubers heal for a day or two before replanting to reduce the chances of infection at the site of the cut.
Because it is now the tail end of the dahlia season, the good news is that there is very little to do until next spring. If you have plants in the ground from this season, consider leaving them in place and mulching over them this winter. Otherwise, you can start brushing up on your varieties and stocking up on stakes.