I'd continue to use ProMix BX or ProMix (or similar) seedling transplant mix. (Home Depot has a bale of growing soiless mix also.) Seedlings will get enough nutrients from these mixes until you get them into your regular garden soil.
I grow Clematis, shrubs, trees plus veggies, perennials and annuals in these mixes. But if you want to add some of your soil in with the ProMix (or other brands) by all means. By experimenting you'll find your favourite growing medium. You will be able to grow on for another year providing they are watered frequently and put into a holding area of sand let's say several inches deep.
If you are just growing them until June 1st then the Soiless Mix is still fine.

sharont~ Perfect timing. I'm chomping at the bit to get started transplanting today and I'm just getting ready to head to town.
Your advice is invaluable and I'm grateful.
Thank You.
Have a great day & happy gardening.

Hi Kanuk:
I remember asking this question when I was first starting but I don't remember what the answer was. Anyway, I do not transplant anything into larger pots, unless I don't have room for them. While in their containers, I occasionally use a very weak solution of 20-20-20 water soluble fertilizer to hold them over but I transplant directly into the soil. They are so small, you think they won't survive but it's amazing how strong they are. If I do transplant, I use a mixture of potting soil and triple mix. I also use that for 'volunteers' which are in the wrong place and which I want to keep. Happy Planting!!

northerner_on
Thanks for sharing. Yes. Some of my seedlings are going straight into their places but I know for sure others are going to be waiting awhile before their rightful places are well prepared. Instead of causing them undo stress by keeping them crowded & in too little soil I'm going to transplant them into larger pots until then. This should help to lessen their pots from drying out as quickly and give them more room to grow on... until .. (cough)...I'm ready!! lol
I am overwhelmed by the success of germination. I stumbled upon WS this winter and I hadn't prepared any planting sites previously. Nothing! Nada! I didn't even remotely consider "planting seeds" especially during the winter!! My previous experience with growing seeds was enough to never want to do it again. (Totally unsuccessful) Not anymore!!!
So I have my work cut out for me to prepare beds etc. This is a new rural property that has never been gardened on before ... so I don't even have any open ground to call a 'holding bed'. So transplanting to larger pots will take the place for holding beds for the time being.
Off I go...dig and ...dig some more!!
Happy Gardening to you too.