By default, only crops from the same seed as the original tile are harvested. You can edit the mod's config.json to harvest all connected dirt tiles instead.
By default, crops harvested drop in place. You can edit the mod's config.json to have them auto-added to inventory instead.
By default, diagonal-only joined tiles are not considered part of the same field. You can switch this in the mod's config.json file.
Technical
Requires SMAPI, uses Harmony.
Compatible with Mod Updater for automatic updates.
Code is at https://github.com/aedenthorn/StardewValleyMods.
If you want to complain or ask for help or help me test my mods, you can visit my Discord server.
A list of all my mods for Stardew Valley is available at https://www.nexusmods.com/stardewvalley/articles/895.