They like cold winters (within limits) and warm, dry summers. Tropical, subtropical, and the warmer Mediterranean climates usually don't have the necessary chilling in winter to keep them happy. Without cold enough winters, they don't bloom, get stunted and unthrifty, and gradually fizzle out. If you are close enough to the Andes to get a definite enough winter--at least 2 months with daytime highs less than 12º C--then nitrogen deficiency or poor drainage can cause those symptoms.

Thanks! I have two rhododendrons that are doing well and I thought peonies required similar treatment. The gardener who sells them keeps them in pots, in gorgeous bloom of different colours. He told me they were easy!

Peonies are easy if you have the right growing conditions. I 'think' your low in winter is only 9˚ and that would be at night so to warm during the day. To be honest I'm surprised that your rhododendrons are doing well.

Thank you, Moon grower. Perhaps it's a special veirety of rhododendrons, I don't know. I planted them two years ago in big earthen pots, but I have been told that plastic containers are better. My azaleas are potted as well.

Arcadio keep them in earthenware pots if you can... these breath!

Yes, Moon; anyway I loath plastic pots. But I have read somewhere that alpines etc. need rainwater if they are kept in earthenware pots, to prevent salty sediment. I have no cistern to collect rain water. But the rhododendrons look comfy, so far.

The main hazard with salt in earthenware pots is that it eventually destroys the pot. It normally doesn't bother the plants much, as long as it doesn't build up in the soil, as well as in the pot body. To prevent that, apply enough water to let ample water out of the drainage holes in the bottom. If you use saucers--not always necessary--be sure to suck all the excess water 10-15 minutes after watering them.
Rhododendrons don't need chilling like the peonies do, so they will do fine as long as the days don't stay over 35º C for very long. Many Azaleas can take even higher temperatures, as long as they have shade.

Thank you, Tugbrethil. I don't like to meddle too much creating artificial environments. But difficult tasks are a temptation and I'm stubborn. On the other hand I enjoy my ordinary and common plants when they are skillfully combined. I simply like their being alive there. Even those that are weeds in one corner of the garden, like oxalis, vinca, klerodendron, are welcome somwhere else.