I started some delphinium seedlings indoors several months ago and they are now about 6″ in height. My question is..do I plant them outside now (Oct) or is too late? Thank you very much for any help.
You appear to have started your seeds too late, hence they are only 6″ in height in October. The proper time for sowing seeds indoors depends on the amount of time required to develop a healthy transplant of appropriate size to be moved outdoors. Usually, instructions will be found on the packet of seeds. Once your seedlings started to grow you were probably waiting for them to grow large enough to put outside and now it’s too late to go to the next step.
Whenever you move seedlings outdoors from the house or greenhouse they need a period of acclimatization. This is because the light intensity and outdoor temperature will be completely different. We call this process “hardening off”. One starts by setting the potted up seedlings outside for a few hours at a time in a protected, semi-shady location. Outdoor temperatures should be 7°C or warmer and the time outdoors should gradually be increased. Your seedlings should be planted into the garden when weather and soil conditions permit. Unfortunately, it may be too late to plant your delphiniums outside, right now. There may not be enough time for the roots of your seedlings to get established before those winter frosts are upon us.
However, if you have several seedlings and you want to experiment, try planting half outside, providing them with winter protection, such as covering the roots with a good mound of mulch and/or putting a cover over the plant. For the remaining half you can try to get your seedlings through the winter without putting them outside. This will be tricky. Perennials basically go dormant in the winter months, when outside. You will have to try and provide a similar environment in your home in order to ensure their survival. Find a cool spot, with some light and water sparingly, and hopefully they will survive so you can put them outside next year.
Below are some sources of information for further reading on growing plants from seed:
https://secure.caes.uga.edu/extension/publications/files/pdf/B%201432_2.PDF
https://www.ag.ndsu.edu/pubs/plantsci/landscap/h1139.pdf