This description is a compilation of details and tips found online. References will be provided. Most of the original mod content I did not write and are owned by their respective owners.
Reference: https://forum.wurm-unlimited.net/viewtopic.php?p=112
Wiki: https://wiki.wurm-unlimited.net/page/Better_Farming_Mod
Mod Description by BDEW:
This mod adds area actions for farming, forestry, and gardening.
Length of action is roughly equivalent to doing the same area manually
Skill gain, stamina drain, tool damage, etc. is processed separately for every tile in the area
By default at 50 skills you can do 3x3, at 70 skill 5x5 and at 90 skill 7x7 area (configurable, can add even bigger areas)
Farming
Cultivate - works like normal, use shovel on tile
Sow - activate any container containing stuff that can be planted and click on tile
Farm - works like normal, rake on tile
Harvest - will only harvest ripe tiles, requires scythe for grains. Harvested crops will be auto-combined in inventory to make one big item (even those that can't be normally combined).
Harvest and replant - same as harvest but will use 1 produced item to immediately replant the tile
Compatible with ago's cropmod - extraHarvest config parameter will affect harvest yields
Trees and Bushes
Plant, Prune and Pick sprout - Require sickle active, click the tile you want it centered on.
Harvest - Require sickle or bucket (for sap) active, click the tile you want it centered on.
Trellises
Harvest, Prune and Pick Sprout - All actions require sickle active, click the trellis you want it centered on. Trellises need to be planted.
Planter racks
Pick all - right click a planter rack with harvestable planters, requires 30 gardening skill (configurable).
Plant all - activate a container and click a planter rack with empty pots, requires 50 gardening skill (configurable).
Additional planter items (configurable):
Nettles, sassafras, woad, cocoa bean, nutmeg,
Blueberry, lingonberry, raspberries
All mushrooms
Downloads and source code: https://github.com/bdew-wurm/betterfarm/releases
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That sounds to me like you have an individual seed item activated.
To have the 3x3/5x5/etc sow action you need to activate a containers with seeds in it.