Protecting recently planted from frost?

Do as I do, and make up some simple wire domes, and grab ya cling wrap out of the kitchen wrap it around 3 or 4 times. Leaving a small hole on the top of it open about 150mm for air to enter and escape. This acts a temp thermal glass house/hummi-crib for ya seedlings ! Thus creating a hot pocket around the plants, to keep them warmer than the external temp ya getting.... Your rosemary back off on the water a little, keep them semi drying, will go a lot better, I have found... The basil luv a good drink, too!

Cover with a light cloth and you should be okay. You don't need to construct some wonder domes or anything. You just need something between the plants and the air. If you can't find anything really light, put in stakes taller than plants so they don't get crushed by fabric.

Rosemary and sage are pretty hardy and likely will be OK with a few hours of light frost. Basil is very, very tender and will probably get badly burned, if not killed. If possible, I would bring them into a protected place overnight. Otherwise, I'd cover them with flower pots, tents made out of a few sticks and old towels or something similar.

Good luck!

a good trick is to cut milk jugs and pop bottles to cover them up and work like a mini-greenhouse until the danger of frost passes. Or if you need something quick in a pinch, take a plastic sheet or tarp and gently lay it over them, even using sticks or poles stuck into the ground to keep the weight or it from crushing the plants' delicate stems. Hope this helps!