i'm not sure if this is climate related or not as i do think they are a tropical plant so they may rot just because it got too cold for them or they froze or something. not sure. i'm definitely not a sweet potato growing expert... or even a potato growing expert... i'm loving it that you're just coming into the main part of summer there now - i get to feel warm vicariously...I learnt that sweet potatoes are Not like normal ones, you cant leave them in the ground expecting them to regrow the next year- they just rot.
You can leave them in the ground, if the climate is warm & the ground never freezes. When I lived in San Diego, a canyon was just outside our back yard fence. DW & I planted sweet potatoes on the rim of the canyon as edible landscaping (a variety grown for their edible leaves). We had to water them sometimes, but other than that, they were exceptionally hardy. San Diego does get an occasional frost, and when that happened all of the top growth died. But when the weather warmed, the plants resprouted. They were still there 5 years later, when we moved away.I learnt that sweet potatoes are Not like normal ones, you cant leave them in the ground expecting them to regrow the next year- they just rot.