In part it is normal for the older bottom leaves to die and fall off. But is usually with much older plants than these appear to be.

Over-watering/overly wet soil is the most common cause of yellowing such as in your picture.

Excess N may cause a similar condition when very high N fertilizers are used but your fertilizer is not high N. Plus with excess N the younger leaves would be very dark green - yours are not - and here would be green veins in the yellow leaves - yours are not.

Too little N would show up most in the new leaves as yellowing.

So consider it a possible over-water issue. Excess rain? How often are you watering?

Dave

They were staying kind of wet with the mulch on there and that's another reason I took the mulch off. I was worried about them staying too wet and not getting enough oxygen.

When the mulch was still there, I was watering maybe 2x a week. I took the mulch off about 2 weeks ago and the combination of that and us finally getting cooler/drier air here in central FL, I've been watering probably every other day when needed (if i stuck my finger in and felt dry).

Very possible they were staying too wet with the mulch but I've been careful not to overwater them since taking the mulch off.