Here's a mod where I attempted to make farming more difficult, if not a bit more realistic while mantaining the original DF crops.
Here's a list of implementations:

- Made all underground crops growable year-round, although with far greater growing times that can span over a year
- Made all above ground crops growable in specific seasons, taking from 2 to 8 months to mature
- Organized the growth times to allow for a very specific interval for planting the above ground crops, otherwise they will be lost on the season change.
- Farming Calendar included to ease farming.

Known issues:
- Bushes are more erratic, often many of them die after each season
- Requires more micromanagement of the plots to avoid failed crops that would waste seeds
- Balance issues?

Installation:

Just replace the .txt file on your raw\objects folder. The .png included alongside is for reference on the appropriate growing season.

Playing instructions:

Testing proved that setting the plant to be grown only in the season marked with a white "P" on the calendar allows for a minimal fuss and less seed wasting, although if your dwarves plant the seeds on the month after the planting season, those plants will fail to mature in time. A good group of experient farmers and a neraby seed stockpile is advised. On the first year you should focus on gathering seeds for above-ground crops and building the farming area, since most of the crops are planted on spring, it'll be hard to have them growing in time. Underground crops now grow very slowly, making underground farming an even greater challenge.

Here's a per-crop breakdown:

Underground Crops:
Available all year round.

Cave Wheat - 16 months
Pig Tail - 15 months
Plump Helmet -12 months
Quarry Bush - 11 months
Sweet Pod - 13 months
Dimple Cup - 14 months

Above Ground crops:

Refer to http://img97.imageshack.us/img97/6937/seasons.png