Question
QUESTION: When and what kind of fertilizer do I put on my Clematis. I have some 16-16-16 and some 20-12-8 both are slow release granules.

ANSWER: Hi Don,
Thanx for your question.  Use a fertilizer high in phosphorous on your clematis.  Fertilizers are identified by three numbers.  I recommend a 10-20-10 or a rose food which is higher in phosphorus than in nitrogen.  Phosphorus is the middle number, nitrogen is the first number and potassium is the last number (N-P-K).  Apply first in the spring in April, then in June and finally in August.  Three times a year.  Too much nitrogen will result in lots of green growth and few blooms.  Any rose food can be used to fertilize clematis.  I hope this helps.
Tom

---------- FOLLOW-UP ----------

QUESTION: Both have leaf buds now and will be fully leaved by April I think. I'm not trying to second guess but would March 1 be better. Stick to your guns and let me know if that is too soon. Also do you know about Grapes? Do they need fertilizing also?

Answer
Hi Don,
thanx for the followup.  Sometimes I forget to ask people where they live and that gives me an idea of weather as I have lived in many parts of the country.  If your plant is actively growing now or will be in March, go ahead and fertilize.  Don't fertilize when the plant is still dormant.  As far as grapes go, yes, they would benefit from fertilizer.  I'm not an expert but I have about 25 plants I just started last year.  Use a balanced fertilizer like 10-10-10 or 12-12-12.  Plain old composted manure would work.  Here is a nice reference from the University of Missouri I think you will find helpful.
Tom

http://extension.missouri.edu/explore/agguides/hort/g06085.htm