why is my 5 year old rhubarb plant not getting tall?

Hi Britt and welcome to GoY. Have you fed it at all in the past five years? Something like well rotted horse manure or other organic matter? If not then it is probably hungry and will need a good feed this autumn.

Is it in the ground? If so, Moon Grower's answer is applicable. If its in a pot, that's a different matter...

I too am interested as mine is only in it 2nd year..

You don't need to split rhubarb unless the crown gts congested, which it won't in two years.

Snoop in case Bamboo doesn't see your question, I think she meant that most pots would be too small - rhubarb is a vigorous hungry plant and needs to be in the gorund.

Yep - doesn't do that well in pots, though the RHS says it can be grown in a pot that's 24 inches wide by (I think) 12-18 inches deep.

You'd seriously need to feed if in a pot however big.