Do some varieties of rhubarb have little or no red on the stalk?
Yes, there are green stemmed varieties. You can treat them the same as the red stemmed varieties.
My rhubarb has grown a flower stalk. I was wondering if I can harvest those seeds and plant more plants?
Yes, you can harvest the seeds for propagation if you would like.
What is the proper amount and type of fertilizer to use for rhubarb?
Fertilize each plant with a handful of a 5-10-10 fertilizer in the spring at the time of planting or apply manure, compost, or a low nitrogen fertilizer to the soil surrounding the plants.
Planted rhubarb behind our garage. It gets morning sun but not full sun. Should we move to full sun? It is a moist area but after first pulling, it seems to not grow. What fertilizer should we use? Just put some horse manure on the plants--but maybe needs something else what?
These plants typically thrive in full sun but may tolerate partial shade. Generally, working in some compost or manure during planting should be sufficient enough as far as the fertilizing goes. The plant does need well-draining soil, so if the area is too wet, this could be your problem. I would either improve its drainage or move to a more sunny, less moist location.
This article may help: https://www.gardeningknowhow.com/edible/vegetables/rhubarb/grow-rhubarb.htm
How do you tell if the rhubarb is too woody to use?
There is no clear answer to this. It is more of a personal preference. Generally, if the stalks are hard to cut or stringy when cut, then they are too far woody to be usable.
We had a very early spring in Mn and a frost. Can rhubarb still be safe to use after frost?
Rhubarb is a fairly hardy and frost tolerant plant. If it is hit by a hard frost or freeze in late spring, it still can be eaten as long as the stalks are still firm and upright.
My rhubarb came up great this spring, but after the first cutting it is dying off. This is the second year this has happened. What is wrong with it?
It may be over-harvested. Over-harvested rhubarb is far more susceptible to pests and disease. You may need to let it sit for a year and not harvest to let it really establish so it is harder to over harvest it.
But, for this immediate situation, it sounds like a fungus is attacking it after the harvest - so you can try treating it with a fungicide. Neem oil is a good choice as it is organic and safe for consumption.