I have never heard this phrase used before, but I think it means they have a lot of work to do.
In a garden crops are planted in rows. A farmer would use a hoe, which is a tool with a handle like a rake or shovel but the tool on the end is a hard metal edge bent in a 90 degree angle meant to scrape and push the soil. It makes sense that a "big row to hoe" would mean a lot of work to do. However, there may be some other details, subtle nuances, about the analogy I am missing because I have never actually hoed a big row before.