It is a little low. If you ever wonder how low of a temp can yeast handle, goto the manufacturer's website. They will give you all the info on your little yeasties. I just looked on Wyeast website and here is an excerpt right from it.
YEAST STRAIN: 3068 | Weihenstephan Weizen™
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The classic and most popular German wheat beer strain used worldwide. This yeast strain produces a beautiful and delicate balance of banana esters and clove phenolics. The balance can be manipulated towards ester production through increasing the fermentation temperature, increasing the wort density, and decreasing the pitch rate. Over pitching can result in a near complete loss of banana character. Decreasing the ester level will allow a higher clove character to be perceived. Sulfur is commonly produced, but will dissipate with conditioning. This strain is very powdery and will remain in suspension for an extended amount of time following attenuation. This is true top cropping yeast and requires fermenter headspace of 33%.
Origin:
Flocculation: low
Attenuation: 73-77%
Temperature Range: 64-75° F (18-24° C)
Alcohol Tolerance: approximately 10% ABV
Styles:
Dunkelweizen
Fruit Beer
German Hefe-Weizen
Roggenbier (German Rye Beer)
Weizen/Weissbier
Weizenbock