I'm hoping to grow various cool-weather vegetables (primarily kale, chard, and lettuces) this winter in my raised beds here in Seattle and have the option of covering them with clear-plastic hoop houses. I've had some success with overwintering such plants without any covering, so am not sure how much of a boost hoop houses provide.

If winter veggies do grow better with them, when is the best time to put them up? Any other pointers for a winter-veggie hoop-house newbie?