Hi, I live in the Seattle area (not a lot of hot days), and my delphiniums are spindly and short, and the leaves toward the base of the stalks tend to brown and fall off. They do bloom, however. They are planted in full sun near a pine tree. They're about two or three years old. Some are Pacific Giants, and the others are of unknown names. Any clues or suggestions as to why these plants are not thriving? Thanks!
Delphiniums are very greedy perennial flowering plants and they require good growing conditions with as much humus you can lay your hands on to help feed, let air into the soil, hold onto some moisture and allow the roots to spread out a little to enable the tall flowering stems to support themselves.
I would suggest the browning of the leaves could be due to not enough water being taken up bey the roots and maybe this is made worse by the Pine tree taking the moisture before the other plants get the chance, pine trees to my knowledge grow in an acidic soil and there needles add to the acidity so perhaps the Delphiniums are suffering from that too, though as you say they are flowering, they are obviously having to really struggle to get to that stage.
I would offer a good drink and add some liquid feed for this season and next year or even end of season this year, dig up the plants and either replant them in same situation BUT with a lot of added humus by way of good quality compost, or as stated before, well rotted humus by getting free well rotted horse manure, the latter is the best and most stables will allow you to take as much as you want for free, they have the stuff on tap every day. Best time end of season is when the flowers have died and the foliage is turning brown, cut this away and lift the plants causing less damage to the growing cycle as the plants are going into winter sleep.
Make sure any manure is ready for garden use, it should have no horrid smell, when held in hand it should look like nice brown compost and crumble between your fingers and thumb, you can also use this as a mulch each spring or end off summer as you tidy up the borders / beds.
The mulch will also help prevent water evaporation in hot weather and if possible the mulch should be several inches deep BUT, dont lay it right up against the plants foliage / stems.
Hope this can help you out a little and your flowers will soon recover, make sure you keep an eye on them for slugs / snails as Delphiniums are like a treat to those pests.
Best regards. WeeNel.