I am wondering if I need to prune my Feather Grass (pony tails) or Russian sage back, and when is the best time?
Early spring is a good time to prune these plants. Cut back the dead plant growth to about 6 inches or so from the ground.
Is Russian sage edible?
It is not an edible plant, really. The flowers used to be eaten, but the leaves are definitely not edible.
Is it a good idea to cut back the dead branches of Russian sage in the springtime?
Yes, spring would be the ideal time to trim the plant back, especially any dead or unhealthy growth.
Russian sage is taking over my garden. Can I use Roundup in new growths or will it kill main plant? Or how else can I control it?
Before resorting to chemicals, try hand pulling as much as possible. Then you can pour boiling water on the remaining plants that are becoming 'weedy'. Be aware though that this will kill any plant the hot water also comes in contact with. If you still want to give Roundup a shot, simply use a shield of some kind (cardboard works well) to protect the main plant and spray the weed killer on the plants you want to get rid of.
How can I keep a Russian sage plant from spreading? New growth is popping up feet from the original square garden plot intended. IF I use a weed killer on the new shoots of Russian sage that are in spots I don't want, will that kill the entire plant? Is it better to pull the new sprawling plants out of the soil?
Just clip off the unwanted plants.
Using a herbicide will kill the main plant, likely and the risk of accidental exposure to other plants is high.
I don't have any russian sage plants, but our neighbors do and several common areas in our neighborhood do. Can I take some plant shoots from the spring pruning and grow a couple new plants?
You can take a root cutting at really any time but if you would like to start with cuttings, do so in the spring.
Here is a link with more information.
Can I harvest seeds from my Russian sage for next year? I love the low maintenance of this plant in my dry soil. How do you find the seeds, and when do you plant them again?
The easiest and most common way to propagate Russian Sage is through division or cuttings in the spring.
You can however collect seeds.
When the flower sepals turn brown and dry, the tiny seeds are about 1/32 " in diameter.
You do not have to separate the seeds from the chaff but you will need to give the seeds a cold period.
This is called stratification.
Here are some links to help you.