I have some petunias in pots on my terrace for about two months now and they're starting to look a bit wilted even though I water them almost every day. It's brutally hot here in Madrid these days and I'm not sure if it's normal for them to deteriorate so quickly or if I'm doing something wrong.
Daily watering is sometimes counterproductive because people assume more water = better, but the reality is that with Madrid's heat, what happens is you're watering superficially and the water doesn't reach the deep roots properly. Instead of watering every day, try watering less frequently but thoroughly, letting the soil dry out a bit between waterings. That way the roots are forced to go deeper and they handle the heat better. And yeah, the thing about pots heating up is real, but it's not so much that they burn the roots as that they speed up evaporation, so the best thing is to surround the pots with something that shades them at midday - could be white plastic, aluminum foil, or even putting them under an umbrella.
Location is key. They don't need direct sun all day in July in Madrid; getting sun in the morning or at sunset is plenty, and during peak hours (12 to 5pm) they're better off in the shade or with filtered light. If you have them on the terrace with nothing protecting them, that's a big part of the problem right there.
Another thing: make sure your pots have good drainage at the bottom. If water doesn't drain well, even if you're watering correctly, the roots rot from too much moisture (which is the opposite of what seems to be happening to you, but it's worth checking). And if you see they're still declining anyway, a little flower plant fertilizer every two weeks gives them the energy to handle the heat stress.
With that Madrid heat in July, it's totally normal for them to end up like this, but what's probably happening is that even though you water daily, the water evaporates way too fast and the roots aren't absorbing enough. Petunias need soil that's constantly moist but not waterlogged, and with those brutal temperatures you have to check if the pot drains well and if the soil is drying out in just a few hours. My advice is to increase watering but do it smart: water early in the morning or at dusk (never at noon), and put the pot in a saucer with a bit of water so it acts as a reservoir. Also check that the soil is good quality; if you see it drying super fast, it's probably too poor or compacted.
Besides watering, the excessive heat on a terrace with no shade is a killer for petunias. Try to place them where they get sunlight but with some shade during the hottest hours (roughly between 12 and 5 pm), even if it's just partial shade. If you can, add bark or mulch on the surface of the pot to keep the moisture in and lower the soil temperature. And check if they don't have pests, because with the heat spider mites show up really easily; if you see yellowish leaves or weird spots, give them a good misting with water or use an organic miticide. With these adjustments they should bounce back, but don't expect miracles in two days; give it a week or two for them to recover.
I've had petunias in pots for years and what's saved me the most is putting them where they get sun but with some shade during the harshest hours of the day, because in Madrid that July heat literally burns the flowers even when the soil is wet. Also, pay attention to the pot size: if it's too small, the water disappears in seconds and the roots don't have enough soil mass to hold it, so consider moving them to something a bit larger if you haven't already.
What nobody mentions is that plastic or light-colored clay pots get crazy hot with that direct Madrid sun, and that actually burns the roots from the inside even if you water properly! Try moving them to thicker ceramic pots or at least wrap the ones you have with some light-colored fabric to insulate them a bit from the heat. Also, if you can, water them early in the morning or at sunset so the water has time to be absorbed before evaporation goes wild, not at midday!
Don't worry about that thing about watering every day if you're not making sure the water is actually reaching the roots. When you water just the surface in that Madrid heat, the water evaporates before it can sink in, so the plants literally die of thirst even though you see wet soil on top. What works is watering less often but thoroughly, until water comes out of the drainage holes, and doing it during the coolest hours (early morning or late afternoon).
The other key factor is where the pots are. If they're getting direct sun all day in July, the roots are literally cooked by the heat of the container, not just by the air temperature. Find a spot where they get light but are protected from the most intense sun, say between noon and 4 PM. And if the pots are plastic or light-colored terracotta, think about switching them to something darker or wrap the ones you have with something that shades them, because that thermal insulation makes a huge difference.
Also, in that heat move the pots away from really windy spots if you can, because it speeds up evaporation. A light mulch on the soil (something that doesn't smother the plant but keeps moisture in) helps a ton too. Petunias can handle the heat, but their roots need to not be roasted.
Log into answer.
