A few years ago we asked, via a few garden lists, for methods of growing delphiniums in the heat, employed by gardeners in the USA & Canada. You turned out to be a resourceful lot! We also received some good information and growing tips and recently an excellent summary of advice for those in Zone 6. And you can click here for a map that shows where in North America you can grow our delphiniums.

The feedback may be summarised as follows:

Perennial Delphiniums are grown successfully in all except the hottest and most humid areas of the USA. They get a bad press from Texas and no press in Florida.

Mulch

Much emphasis was 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 grew delphinium successfully where summers get very hot (95degF+) all used some sort of shading to protect the plants from the worst heat of the day. This shading usually took 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.

Watering

General agreement here too. Most folk water every day during hot weather. I would add a cautionary couple of notes here. 1) Never let delphiniums get either bogged or dried out (particularly when growing well) or they will die. 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

Delphiniums are generally very hardy. They are grown without trouble in Alberta, temperature range –35degC to +35degC. Winter mulching benefits here and folk often use pine branches to provide a loose protection from severe frosts. We have reports of them going rampant in South East Alaska, growing to two stories high and being tended through the upper level bedroom window.

Types

The most common type grown was "Pacific Giants" as this is the most readily available. General consensus was that the English Hybrid types (such as ours I’m glad to say) were more robust and lasted longer

Species

D. belladonna bellamosum and other belladonna types such as Connecticut Yankees do well in most places. Gene Bush <genebush@munchkinnursery.com recommends the following in the situations he describes. "Delphinium tricorne is an easy and reliable perennial blooming in violet to deep purple that is ephemeral. There are also white blooming types to be found. The white usually contains traces of violet and/or pink. D. exaltum is a tall species with light to dark violet-blue blooms that is perennial and withstands the heat as well as being cold hardy. D. carolinianum is from Virginia to Florida and over to Missouri and Oklahoma so that would make another good candidate."

Comment

The following are comments from a few of the respondents to our questions. I would like to thank these kind folk for allowing their notes to be published on our site, and to thank everyone who took the time to respond.

• From Gayle Fields, Alberta: Delphiniums grow in Lethbridge, Alberta where the temperatures range from -35C in the winter to +35C in the summer so you shouldn't have any difficulty. I cut mine back after their first bloom and then get another bloom in the fall. I grow Blue Mirror and am going to try "Red Rocket" this spring. They like a bright sunny place and grow about 4 to 5 feet high. Sorry for the mixture of metric and imperial - I am of the age where I am only partly converted!
• Peggy Enes Kansas: I live in a Zone 5/6 US hardiness zone (NE Kansas). We also can have some really evil summers with temperatures going 100°+ some years. Although a fairly new gardener, I had good luck with the little Blue Butterflies last year with *heavy* mulching and regular watering (drip irrigation) although they flagged a little above 90°. We did not have 100°+ last summer but had long stretches of high humidity. The butterflies were grown in my very nasty, completely depleted, *heavy* clay soil with 1/3 cotton burr compost and 1/4 sand amendments (1st year). They were mulched (4-6") with coca shells and regularly watered (we had a summer with practically no rain) so I can't speak about drought-resistance. I fertilized with Osmocote (a 3 to 4-month timed-release fertilizer) since my soil is still a few years short of being adequate to support much but weeds.
• From: "Carolyn Zard" Washington State Our summers in the Pacific NW are not hot - even if we do get into the 90's it cools off at night because of the maritime air surrounding us. The only suggestion I have is to practice good garden culture to keep your soil & plants healthy and be sure the soil is moist going into a hot spell. The British cultivars I've grown the last 4-5 years have not suffered from any weather conditions except the wet Spring in '97 that greatly reduced their bloom time and seed ripening. They're pretty rugged. Carolyn
• Janis, an enthusiast on the Perennial Newsgroup had this to say about growing Delphiniums in Missouri Zone 6 Most of the experts around here say you can't grow delphiniums in the St. Louis area. Well, it can be done, because I do it. I just love them so much that, like you, I have to try. That is really odd for me because my philosophy of gardening is to grow the right plant in the right place. I grow mostly natives and insist on having only plants that don't need pampering. I see my yard as more of an ecosystem than a garden. And we have so many wonderful, wonderful natives to choose from. And yet I make an exception for delphiniums because they are so lovely and I just adore them. They are one of the few plants I consider worth pampering. So I give them lots of special attention. Since the rest of my plants don't need much, I can afford the extra time. They need staking, dividing, deadheading, watering, topdressing, and constant fertilizing. Not a plant for beginners or the lazy. You are right, you should amend your soil for delphiniums. They like rich, organic, moist and well-drained soil. If the soil is not well-drained they will get crown rot. If they are too dry they won't grow or bloom. That is a nice balancing act - moist and well drained. Best done by adding lots of organic matter (I use compost) to the bed and then keeping them watered, but not too much water or they get powdery mildew, etc. Delphiniums prefer slightly alkaline soil. As organic matter tends to be acidic, I also add some lime in the form of calcium magnesium carbonate. After preparing the bed and getting the plants established, I top dress with compost and lime every spring. Delphiniums are heavy feeders. If not fertilized regularly they just sit there and pout, refusing to grow or bloom. The annual topdressing of compost that serves the rest of my plants well is certainly not enough for my delphs. That well-balanced diet of nutritious, homemade compost food is not enough for them. They are fast food junkies! If that were not enough, they are susceptible to lots of diseases. Although, knock on woodys, I have not had any disease problems with mine. And they are often short-lived. I just keep adding a few plants each year, and I have been letting them go to seed after the second bloom, so I always have a new crop to replace any that fade away. You can do all of that, but the biggest problem with growing delphiniums in the midwest is that they just do not like our climate. You can amend the soil, but you just can't amend climate. I always put them in the back of the garden (which is a good idea anyway since they are tall) and try to put them where they will get lots of morning sun and then be shaded from mid-day on in the heat of July and August. I try to shelter them with plants that emerge late and get large by late summer. They really like more sun than just in the morning, but they can't handle the heat here so it is a compromise that must be made. In the hot sun of the midwest they slow down, sometimes even die. (They are much more disease-prone in full sun, too. And I think that is true anywhere.) Have you tried the belladona delphiniums? They do much better in midwest climates, especially Delphinium x belladona 'Bellamosum'. That, and the old- fashioned species delphiniums are much easier to grow here. They have a longer bloom time, and more flowers, but the flowers are smaller. They are fine plants, but not so grand as those Dowdeswell beauties. Some of the big D. elatum cultivars are more heat-resistant, too, like the 'Blue Fountain' strain. I like to fill in with the belladonas and then have a few grand, huge stars as focal points among them. And so, Margaret, if I can grow Delphiniums in Missouri, then you can grow them in Indiana. Have fun, and good luck! --Janis, Zone 6, Missouri