Delphiniums are stately and colorful sentinels for the back of the ornamental border, whose towers of blossoms may be used as cut flowers. Their height and stalk fragility make them vulnerable to strong winds and rains, so many are staked to remain tall. Those usually available in U.S. nurseries are variations of the Pacific Giant series, names evoked by visions of King Arthur’s medieval era. Names such as Guinevere, Astolat, Percival, King Arthur and Black Knight, for instance. Newer varieties, often shorter in stature, don’t need to be staked, and neither do the New Zealand delphiniums.
The center portion of the blossom, where the reproductive parts are located, is called a bee. It could be a different color than the petals. Thus you have a delphinium that’s described as “sky blue blossom with a black bee,” for instance. Technically, delphiniums are short-lived perennials, but high heat might make them more usable as annuals.
If you buy a delphinium or three, look over the soil where you plan to locate the plants. They should be situated where they’ll have morning sun and afternoon shade, if possible, as well as good drainage. Dig in 2 to 4 inches of compost or composted steer manure, digging deeply, 12 to 16 inches deep. If you have shallow caliche (hardpan) in your area, break it up before planting. Dig your hole at least twice the diameter of the potted plant, and to the same depth, for transplant. I soak the holes, let water drain, and then plant delphiniums to the same depth they were in pots. Water generously, and pat soil firm with the back of the shovel. As the plant grows, tie the plant to the stake with soft cloth, such as strips of old T-shirts. Begin tying before the plant exceeds 12 inches. Slugs love delphiniums, so take corrective action quickly after transplanting.
After they blossom, cut plant back to base foliage to encourage another blossom. Delphinium blossoms attract hummingbirds and butterflies, and some reports indicate deer are not attracted to them.
Install a stake next to the new plant before it develops more roots that could be damaged by later insertion of a stake. If you install the stake when transplanting, you won’t worry about forgetting until too late. Delphiniums set seeds, of course, after blossoming, so that would be our autumn, when frost will kill the plant. Seeds are difficult to germinate because they must be very fresh, and they’re not suited for germinating in cold weather. How could we start delphiniums from seed then?
Edwards Greenhouse has been growing English-style delphiniums for several years, obtaining seeds from New Zealand. The seasons there are the exact opposite of ours, so their seeds set in our late winter or early spring, and they can ship seeds to the U.S. just in time for spring sowing. Anju Lucas, head of perennials at Edwards Greenhouse, says their New Zealand delphiniums are identified as either “millennial” or “New Zealand,” and they produce very sturdy stalks that don’t need staking. She said one of her New Zealand delphiniums is 6 years old, and that’s old for that variety of flower.
▪ This is apparently a bad year for spider mites. They thrive in hot, dusty conditions, so frequent misting of residential yards discourages their habitats. You can control spider mites with soap sprays, commercial or homemade. Be cautious about “soap” sprays; we tend to think of all cleansers as soaps, but some are detergents and are chemically different. Many folks use Dawn, for example. It is a detergent, containing an oil emulsifier, that may or may not kill your plant. I’d test a small portion of your plant if you’re using a homemade spray.
Safer’s Insecticidal Soap Spray is a soap, and so are laundry bars such as Fels Naphtha and Dr. Bronner’s. Part of a bar should be shaved and dissolved before use to kill insects. Two tablespoons of the dissolved soap product per gallon of water should be sufficient to kill spider mites, and this treatment can be enhanced by adding a couple of tablespoons of cooking oil to the mix. Apply heavily, let the mixture remain on leaves for two or three hours, and then rinse off. Prepare a new soap mix each week. Do not apply when temperatures are 90 or hotter, lest you burn leaves.
If you plan to use diatomaceous earth (DE) to kill any insects, apply the DE to the soil around your plants to avoid killing honeybees. They spend time on foliage as well as in flowers.
Send garden questions to firstname.lastname@example.org or Gardening, The Statesman, P.O. Box 40, Boise, ID 83707.