Matt wrote:Yep, ditch the dome. There is no need for it. It will just increase the chances of mold starting. I do keep the humidity artificially high for my seeds for a while, but once they germinate, there is no need to. Also, they will germinate just fine without artificially high humidity.
For adult plants, no cover is necessary even if you live in a dry climate, so long as a plant is acclimated to the low humidity. In fact, as others have mentioned, a covering around and over the plants can be harmful. It traps light and heat and can easily bake and kill plants; also, as linton mentioned, the excessive humidity coupled with lack of fresh air can help fungal infections to form that may kill plants. But for seeds--
Like Matt, I keep the humidity high while the seeds are germinating, because I live in a very dry climate and because the seeds are sitting on the soil surface directly exposed to the air. So in my case, I use a plastic container with a lid. But if you live in a more humid environment that shouldn't be necessary.
If you do use a lid or covering of some kind, be sure to open it every day and fan the air above the container for a fresh air change. Also, as Giovanni mentioned, lids can cause heat to build up very fast inside containers, so keep it out of direct sunlight. Bright shade is fine; after all, the plants haven't really started to grow and photosynthesize much of their own food yet, but are using the stored food from the seed in the cotyledons (seed leaves).
Good luck--
Steve