I know fertilizer isn't always necessary to promote growth of purple fountain grass, but if I were to fertilize, what would you recommend?
A relatively small amount of balanced natural fertilizer (for plants, not grass) in the spring is all they need to grow. In fact, compost should be fine. Only give them additional fertilizer if leaves appear yellow and sparse. Adding additional phosphorus (like bone meal) can help if the plant fails to produce plumes.
I just had a bad experience with an ornamental grass that made so many babies that I had a hard time keeping up. I've removed them and wanted to know about purple fountain grass. Does it overly propagate? What other grasses would you recommend? How do I control over-population of grasses. Do I cut the plumes back? Thanks.
While it really depends on the type of grass you have, removing plumes as they develop does often help to prevent seeding and subsequent spreading. As for the fountain grass, you should not have any of these issues. It is not considered invasive at all. Here is more information for growing this plant: https://www.gardeningknowhow.com/ornamental/foliage/fountain-grass/tips-for-care-of-fountain-grass.htm
https://www.gardeningknowhow.com/ornamental/foliage/purple-fountain-grass/grow-purple-fountain-grass.htm
Will purple fountain grass grow on the edge of a lake, just above the natural vegetation that grows out of the water?
As long as you can ensure that its roots do not become too soggy and the area drains well, then it should grow fine as edging near your lake.
We live in eastern Washington and typically have below freezing winter temps. Do we need to lift our fountain grass for the winter? What is the max low temp in winter?
It is hardy to about 20F. But, if the temps there got lower than that, there is a chance the roots survived if the ground did not freeze. I would give it a few weeks to see if the roots start to grow new leaves. If you do not see any growth in a few weeks, it is likely dead.
I have Rubrum in planters in my home. My cats love to eat it. Do I prune it in late winter as if it was outside? Also, do I keep watering it if I do prune it? Thanks for your help.
Clip the foliage down to about 3 inches from the top of the pot and water well. Then refrain from watering except once a month over winter to prevent the plant from becoming too dry. Once warmer temps return in spring, return it outdoors and give it a boost of fertilizer to stimulate new growth.
The foliage has turned brown; plumes are sparse. Is this a seasonal thing or are the plants in distress?
Your Fountain Grass is reaching the end of it's growing cycle and is starting to go dormant.
You can leave the plant as is and allow it to die down naturally.
You can trim back the plant now or when you start to see new growth.
It will began to grow again after it has rested.
Most regions grow the Purple Fountain Grass as an annual but it is a perennial in your area.
Here is a few links for you.
https://edis.ifas.ufl.edu/fp464
https://www.gardeningknowhow.com/ornamental/foliage/purple-fountain-grass/grow-purple-fountain-grass.htm
Best method, if possible, to grow purple fountain grass from a cutting.
Purple Fountain Grass can not be propagating from a cutting.
You can divide the plants, make sure you dig up enough roots for each division.
Spring is a good time to make divisions. Look for the first signs of green growth.