Just a tip, pig tails are brewable but not edible.
As is every other underground crop bar pig tails. Sure, plump helmets are the only brewable crop you can grow in winter, but above-ground crops are all growable all year round.
Getting a greenhouse going even on a supportive biome might be tricky if anybody going outside is likely to be attacked. You can dig a moat upwards with two steps. First you dig an up stair, then channel out the tile above it. The floor tile on the surface is channelled as normal, but can't form a ramp because the stair tile won't support it. From there you can turn the area inside the moat into a greenhouse, although you can't do this with enemies in seeing distance. A temporary measure might be to create something like this:
level 0
,,,,,
,.,.,
,,,,,
,.,.,
,,,,,
level -1
#####
#<#<#
##>##
#<#<#
#####
level -2
#####
#####
##X## <--fort access
#####
#####
And then remove those up stairs. You get four tiles that are open to the air but with limited visibility, and while ranged attackers can shoot in (based on arena testing) and fliers can either wander in or use it as a shortcut should they get a ground path it's less risky than trying to set up a walled area on the surface itself. Placing it in the middle of the map will make wandering squads less likely to stumble upon it, and placing the open-air tiles so that they're surrounded by trees will cut visibility further. Something's going to find it eventually though, so get the channels covered as soon as it's feasable.
As with mood items, this is one of the things best set up in the first couple of peaceful years.