Although I've had many failures growing them, I've made some progress in recent years. Although I've had many failures growing them, I've made some progress in recent years.

Simple past tense does work here, but it seems to kind of suggest that the process of growing them is finished, completely in the past. No present consequences. I had many failures growing them - but now I've finished doing that. For an ongoing process like gardening, I'd be more likely to use present perfect, even for past failures. It doesn't imply the failures were recent, just that you're still engaged in the same process. Or if I wanted to emphasise that the failures were long ago, "I used to have many failures".

But more about ithat next time, maybe. But more about that next time, maybe.

Just a little more natural.

Yet,But they had some problems in the last summer, which manifested in growth stagnation and leaves yellowing. But they had some problems last summer, which manifested in growth stagnation and leaves yellowing.

"yet" on its own is a bit strange here. "Still, they had" or "And yet, they had", or "Even so, they had" work. But good old "but" seems most natural. The second clause is okay, but sounds very technical. Something like "last summer, with yellowing leaves and stunted growth" would be more likely. And "in the last summer" is okay but unnecessary.

However, I saw've seen pictures of dry fields with plants doing really well, so I suspect the soil might have been too wet. However, I've seen pictures of dry fields with plants doing really well, so I suspect the soil might have been too wet.

Again simple past is okay but present perfect is better. There's focus on the action being completed and the result of that, the present consequence - because you *have seen* these pictures, you have a suspicion.