What else would be there if there was no garden? As long as the soil's not touching the wood and rainfall drains away properly there should be no problem.

At my parents' old house I had a garden at the west side and all the parging (stucco for the foundation walls) fell off. I thought it was because of the garden but it turned out all the parging that company did around the same time fell off, so it wasn't the garden at all. :S

I expect that if it is a foundation garden, you are watering? Would this mean periodic splashing of dirt and water on the siding? Even if not, if there is anything to significantly slow air movement or increase humidity, I'd have to agree with your DH.

Speaking from experience, my house used to have Masonite siding (a type of hardboard). On the north side of the house, the previous owners had banked soil against the siding above the cement block/wood frame joint on the end to the east. To the west, the soil was 5 inches below the siding. When I bought the house I immediately dug out the east end, but left the west end as it was. Mowed grass grew right up to the house. Over the years it was evident that the lower edge of that siding significantly degraded while lower edges on all other parts of the house were still fine.

Rick (who now enjoys steel siding)

Masonite - that's the word! LOL

We used to have grass right up to the house and then dh, who hated mowing, ran a strip of rolled roofing along the house and put gravel on top of it. It was okay, and maybe i should have just put containers there, but i slowly over the years took out the gravel and the roofing and planted gardens there. I do water, but not all that much. Occasionally, i'll water along the back edge when it gets really dry. I see no damage anywhere, but it "could happen", you know! In the front we have vinyl siding, so there have been no complaints (yet! LOL).