Dahlias are tender perennials, meaning they will only survive mild winters, specifically for them in zones 9 – 11. How easy it is to grow them as annuals in other climates depends on the combination of length of season, heat, and humidity. Southwest Idaho definitely has the heat aspect of the trio, but is lacking somewhat in length of reliable warm season, as well as presenting the difficulties of a dry climate.
The dryness can be overcome by consistent watering, good soil management, and simple moisture retention methods, such as mulching and micro-environments. For instance, my dahlias have thrived a northeast exposure that brings them lots of morning sun, but protects in the scorching heat of mid-summer. However, they have also done very well enough in the all day sun with regular, adequate water that I will plant them in such places regularly, too. Both of these locations have friable, rich soil from me gradually mulching it over the years.
It is the length of season that is the biggest issue. Dahlias like the soil to optimally be 65 – 70°F to begin to grow. I have overcome this in the past by planting them early in pots in the greenhouse, then harding them off on the patio before planting outdoors. That way I get blooms most of the summer, instead of just a couple of weeks before frost, which is what I would get if I waiting until after the last frost dates to plant them directly outdoors, as suggested. This is labor intensive, particularly when it comes to carrying the growing gallons back and forth each day, so I usually limit it to 15 or so pots.
However, recently I was chatting with another gardener who loves to grow dahlias and has a direct planting method that has worked well for him here in my area. He also recommended dahlias.com as a supplier. I have already put dahlia seeds in my shopping cart and am trying to choose a few tubers, too. I am going to start some tubers in pots as usual, start some from seed, and give his direct planting method a try.
1. He recommends planting the tubers out in March, in a sunny location. The main trick is not to plant them quite as deep as suggested, because this helps avoid tuber rot. The dahlia tubers can tend to rot in cold, wet soil, which is likely to be the kind of weather for spring in southwest Idaho. Being closer to the surface means they will warm sooner and water will more likely stay drained away. By March, it is highly unlikely that temperatures will get cold enough to freeze below the surface of the soil.
Just how deep “deep” is will depend some on the size of the dahlia tubers you get. My main flower gardening reference, The Encyclopedia of Flower Gardening (which you can apparently now get for 35 cents via amazon!), says that tubers for larger (growing to 3-4 feet) should be planted 7 inches deep, with the top portion being about 3 inches underground. My friend said he plants the with the top only about 1-2 inches under the dirt. This helps them stay warmer and drier. For smaller varieties, the suggested planting depth is already more shallow, so planting just a little closer to the surface will not be as much of a difference.
2. When choosing a sunny location, make sure it is full sun, not dappled or intermittent due to other plants. Also, make sure that it will still be in the sun as trees fill in with leaves, and with the summer course of the sun compared to nearby buildings. But, as I mentioned above, an eastern facing location next to a building, may provide a nice amount of sun, plus protected warmth from the presumably heated human dwelling.
In case you are not familiar with dahlia tubers, they are considered a type of tender, thin skinned bulb. The tubers for one plant are like a grouping of small, sweet potato shaped roots all attached to a main base, from where the main stems will grow. They were discovered in the mountainous regions of Mexico by a Swedish botanist, Anders Dahl, from whence their general name is derived. The tubers are apparently edible. They have been significantly hybridized from what was found in the wild, and there are now considered to be over 20,000 varieties in a range of sizes.
My preference has always been the taller dahlias with the large, exotic blooms. The shorter, hybrids seem a lot like other low growing flowers and I have other favorites for that category, like gazanias and portulacas. I usually plant one large dahlia in the center of a pot by my front door, and lower or hanging flowers around it. I also make sure there are dahlias clearly visible from the main windows I look out of while doing chores. There is nothing quite like a dahlia to add an elegant dazzle to your summer day.