I can't seem to get them to live. They are in the sun. I water daily and I even put straw around the base.
These are particularly difficult flowers to grow. Most will avoid them because of this. However, with the correct care, you can get them to grow. This article will guide you on what they need: https://www.gardeningknowhow.com/ornamental/flowers/delphinium/growing-delphinium-plants.htm
The plants are now about 4 inches high with plenty of leaf. What should I do to keep them for next year - greenhouse, plant outside but will they survive the Winter? Keep them in the greenhouse or keep them on a sheltered balcony after re-potting in larger pots, as they are in the very small ones at the moment. Katherine
If you are located in England where winter temps hover between 30 and 40 degrees F ( -2 and 8 C) then you should be able to plant them in the ground and they will survive winter. I would mulch well, though. Choose a planting site that is moist and well drained and is protected from hot afternoon sun and winds.
https://www.gardeningknowhow.com/ornamental/flowers/delphinium/growing-delphinium-plants.htm
https://www.gardeningknowhow.com/ornamental/flowers/delphinium/delphinium-winter-care.htm
Since your hardiness zone 9b can get below freezing in winter you will need to bring your plant indoors or overwinter in a greenhouse. You do not need to fertilize during the winter. Here are more tips:
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/growing-a-dipladenia.htm
This article will help you to grow larkspur: https://www.gardeningknowhow.com/ornamental/flowers/larkspur/growing-annual-larkspur-plants.htm
Although you don't want to plant things in the same hole, you can plant some things nearby. Here is an article that will give you examples of companions for larkspur: https://www.gardeningknowhow.com/ornamental/flowers/delphinium/delphinium-companion-plants.htm
why are the buds on my Blue Delphinium not opening? It has a lot of buds for a while but none of them are opening
This plant is notorious for being a little picky. Once the correct environmental conditions are provided, the plant will flower, normally.
This article will help you to know what they require:
https://www.gardeningknowhow.com/ornamental/flowers/delphinium/growing-delphinium-plants.htm
Over the past few weeks I have noticed my delphinium leaves (I have 5 plants) turning brown and black, curling and falling off, as well as some of the flower buds. I have removed the dead and sick leaves and buds and flowers (which have finished for now anyway). There is new growth at the base of each plant that looks healthy. I had a good look today and found these brown larvae underneath one of the leaves (see photos). What is it? What can I do? Should I cut back each plant - how far down?
I am unable to identify that type of larva. I have never seen those, here, in my area. I would try a general purpose pesticide. Most organic methods are broad spectrum and will treat a large number of insect pests.
Here are some articles that will help:
https://www.gardeningknowhow.com/special/organic/what-are-organic-pesticides.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides
https://www.gardeningknowhow.com/ornamental/flowers/delphinium/growing-delphinium-plants.htm
I bought some (plug) pacific hybrid delphiniums which did not arrive on time due to Covid - 19 delays. Consequently, they were late started in pots and were not very strong specimens on arrival. Fast forward to August, and the plantlets are looking strong, but very immature, only 5 inches or so tall, so look unlikely to flower this season in the Midlands of the UK. Is there a way to overwinter them to plant them out next Spring? Thank you.
Your zone is equivalent to our USDA zone 8. This is just a little cold for this type of Larkspur to be a Biennial in your area. This situation might warrant a container grown plant, this time around.
Fortunately, these plants are not difficult to grow in container, which will allow you to bring it inside as the weather cools.
This article will help:
https://www.gardeningknowhow.com/ornamental/flowers/delphinium/growing-delphinium-plants.htm