Crop Fertility is a key feature in Serene Seasons.
Throughout the seasons, the types of crops you can grow will change. Crops can be set to grow in any of the temperate seasons, and in biomes affected by tropical seasons, only Summer crops will be in season. Likewise, biomes that always receive snow will only be able to grow Winter crops.
You can see which season(s) a crop is fertile in the tooltip that appears when you hover over the seeds for the crop, or in some cases, the crop itself.
When a crop is in season, it will grow as it normally does, and you'll be able to use bonemeal to grow it faster like normal. When a crop is out of season, it will grow very slowly, and will require more bonemeal than usual to increase its growth stage. To grow out of season crops efficiently, you have three options:
1. Placing glass/stained glass blocks within 16 blocks above crops to grow out of season crops above ground.
2. Using bees. Bees will pollinate crops regardless of whether they're in season or not.
3. Growing out of season crops underground, as long as they're below a certain Y level (48 by default) and not exposed to the sky.