My cousin is having a noon wedding ceremony in a botanical garden and an evening reception (beginning at 5 p.m.) at a country club. My question is: Do I need to wear one dress to the ceremony and a different, more formal dress to the reception??
Probably yes. It is difficult to know for sure though. So, you could ask what the dress code is for the country club. If it is evening resort wear, you may be able to wear the same outfit to both. But, I really doubt it.
Probably you will have to wear cocktail attire for the reception.
Sounds like the reception is going to be very formal, so you definitely want to be in your best attire for the reception. As for the ceremony, I am sure you won't be the only guest there wearing the same outfit to both. So, if you don't have another that you want to wear to the ceremony, then just wear the one that you will be wearing to the reception. However, if you have a nice dress that is a little more casual (not, too casual...this is a wedding afterall) then wear that one to the ceremony at the botanical gardens, and then change before the reception since there is a large time gap between the two.
If you do wear a semi-casual dress to the ceremony I would suggest to go with one in a great summer color. This way you will look bright and beautiful in a gorgeous atmosphere with brillant vibrant colors. You will blend into the garden nicely.