Where you are at I think it would be very wise to diffuse some light thru the roof. Our big conservatory here in Columbus, Oh uses frosted glass even for our zone. The horticulturalist told me plants will definitely burn if near or touching the glass. It must not cut too much light for they have a huge and wonderful fruiting Breadfruit tree as well as tons of other tender tropicals. They also use a "paint" to put on some of their glass sides to diffuse the light somewhat for summer. The first freeze causes the paint to dissolve and flake off allowing more light in for the winter. I don't think you'd have any issues with the amount of light down there. The Whitman Pavillion at Fairchild Gardens have a polycarb roof which diffuses the light as well. Depending upon your application, it may be easier and less costly to use polycarb instead.