What part of March?
We've been there then a couple of times. One trip, it was the start of F&G, it was below freezing at night and miserable during the day (much like this past weekend was).
They had to cover/wrap all the plants. Can you imagine what it was like having to wrap all the topiaries at Epcot?
But the later in the month you go the warmer it gets, the less of a chance of freezing temps.
But PP is spot on, anyone who tells you what the weather will be is either a, pulling your leg or b, someone you need to get lottery numbers from.
My standing advice to someone traveling from mid-Nov on through to mid-March, pack for both 40 and 80. You can have both temps in a 24 hour period.