Sweet potatoes

I am living in northern Spain (Barcelona) and I have planted some sweet potatoes in April. I have now nice purple flowers and leaves are a bit yellowish. Does this mean they are ready? If not, how to know when they are? Thanks.
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Sweet potato is a tender perennial. Here we set the plants in April and dig in October. However if one is in a hurry, there will usually be roots large enough to bake by mid August. If you don't wish to kill the plant just gently dig around the base and remove one or two of the largest potatoes without destroying the main root. If you have frost, the idea is to remove the main crop before frost.
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I have no idea why you are getting brown leaves. You normally don't have to use fertilizer on them. Have you done a soil test on your garden? I raised them nicely in soil that was slightly acidic. Now I live in a state that has soil that the pH is nearly neutral, and they don't get as big. I don't know if that is the reason, or because the growing season is shorter now.
Don't over water them. The trick is to water them evenly and as often as needed to keep the roots damp. If you water uneven, they will grow, stop, and then grow again which will cause cracking on the tuber.
I agree about waiting until just before the first frost to dig them. Here I dig them around the end of Sept, cure them, and store them for the winter. I still have a lot in my celler from last years crop.
If you would like more information about them on curing them before storage, or starting the slips for spring planting, contact me directly.
Have fun. Dwayne

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