I planted petunias in my garden back in May and they took off well at first, but now in September they're just all leaves and barely any flowers. I watered them properly, fertilized them with that NPK fertilizer I bought at Leroy Merlin. Could it be a nutrient deficiency or is it just too hot here in São Paulo?
Petunias really do like a lot of sun, but here in São Paulo in September the heat gets way too intense and they can get stressed out and stop blooming. Try watering more often (the soil can't dry out completely in this heat) and give them some partial shade in the late afternoon to give them a break - like about 30% shade, nothing that blocks all the sun.
Are you watering the soil at the end of the day or during the midday heat? Because in September here in SP the soil dries out super fast, and if you're watering during the day the heat evaporates the water before it soaks in properly. Try watering early in the morning or late in the afternoon, and check if the soil is actually moist about 5cm deep. Petunias in extreme heat sometimes just go dormant and start flowering again when it cools down a bit.
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