|I'd like to cover a gate behind a large Hosta with moonflowers and morning glories. How big should the seedlings be before transplantation, they are now about 1|
|Usually, these plants are started by planting the seeds in the place where they are to grow; the seeds are planted a week or two after the last frost so the soil has had time to warm up. They can be transplanted but they are somewhat "touchy" about it because the root systems are sparse and so the potting mix falls off; the vines try to grow like crazy and quickly get too big and unmanageable in containers and, most important, indoors they just do not receive adequate light.
The transplants need warm weather plus rich, evenly moist, warm soil in order to grow their best. They bloom best in full sun. If you put the seeds or plants into cold soil, they may not survive.
If you start them in containers, each vine would need a good sized container, at a minimum a four inch pot but preferably larger, plus a stick inserted into the pot to climb on, and should be in the brightest possible light.
You will need to condition them so they can adapt to the real sun outdoors, this takes a week to ten days of gradually increasing amounts of direct morning sun. If your last frost date has already passed you could begin acclimating them now so you can plant them outside as soon as possible before they outgrow their containers. Keep them in a protected spot and bring them inside or cover them on cold nights.
I would put them at the base of the fence and they will find their own way up onto it, they are basically pre-programmed to grow up and grow by twining. You can put a few twigs or sticks leaning over them toward the fence to help them locate it. They could strangle a hosta by growing over it. Also, if you plant them in front of the hosta they may try to grow toward the light (and away from the fence) in their search for something to climb.
Good luck with your vines!