Growing Delphiniums in the USA or Canada

We often receive comments from gardeners who grow our delphiniums saying how wonderful they are and telling us what methods they use for growing them successfully in the USA & Canada.

Below is a summary of advice for perennial delphiniums growing between USDA zones3 and 8. We’ve even had a success story from someone growing them as annuals in Florida.

Click here for a map that shows where in North America you can grow New Millennium Delphiniums.

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General Comment

New Millennium Delphiniums (NMDs) are grown successfully as perennials in all except the hottest and most humid areas of the USA, but even there enthusiasts grow them as annuals.

Mulch
Much emphasis is placed on mulching. This should be quite thick, maybe three or four inches deep or greater if very loose (like straw). The mulch helps to keep the roots moist and cool (most desirable)

Heat
People who grow delphinium successfully where summers get very hot (95degF+) all use some sort of shading to protect the plants from the worst heat of the day. This shading usually takes the form of trees which cast a shadow over the plants. The tree could be quite loose leafed. Adjacent trees help cool the ground too. If growing as annuals plant at the end of summer and flower before the heat of the following summer.

Watering
General agreement here too. Most folk water deeply every two or three days during hot weather (some even daily) and maybe once a week at other times. The key is to water deeply. I would add a cautionary couple of notes here. 1) Never let delphiniums get either bogged or dried out, particularly when growing well. 2) Keep only barely moist immediately after cutting down after the first summer flowering (July) or they may rot. Increase water as the plants grow back. Be sure the soil is moist going into a hot spell.

Cold
New Millennium Delphiniums (NMDs) are very hardy. They are grown without trouble in Alberta, temperature range –40degC to +30degC. Winter mulching benefits here and folk often use pine branches to provide a loose protection from severe frosts. We have a report of them going rampant in South East Alaska, growing to two stories high and being tended through the upper level bedroom window. This is unusual however. Incidentally, they grow magnificently throughout Russia too.

Height
NMDs normally grow between three and six feet tall according to methods and varieties used. Those gardeners who give the plants plenty of space and water lightly restrict their height to between three and four feet. Gardeners who plant more closely together, water, feed and mulch well get taller plants.

Feeding
Gardeners all agree – the more you feed them the better the plant. Animal manures seem to give the best results but people often use a general fertiliser from the local garden store. We receive regular comments about how strong stems are and how green the foliage is – much more so than Pacific Giants

Types
The most common type of delphinium grown is "Pacific Giants" as this is the most readily available. General consensus is that New Millennium Delphiniums are far more robust and last much longer in the garden as well as giving a much better show. This is why the famous Butchart Gardens in Victoria on Vancouver Island BC, Canada and Walters Gardens, the largest wholesale specialist supplier of perennial garden plants in the USA, now grow and stock only New Millennium Delphiniums.

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Dowdeswell's Delphiniums - Wanganui, New Zealand