1. The presumption that container gardening means patio gardening, or at least that you have a large terrace. If you can use enormous containers, raised beds and so on, its a whole different ball game from balcony gardening, and much of what you need to know is no different from gardening with a garden. The book often looks great, but relegates small container gardening to hardly more than a chapter.
2. The perfect pictures. Five different types of plant in the same container, all in flower and at their best, and all exactly complementary heights and lengths at exactly the same time. Yeah, yeah. Buy them from the garden centre, give them a week to settle and they'll be like that - for another week. But then one will shoot up, another will get attacked by pests, and a third will stop flowering. And the container will spend the rest of the summer looking tatty.
3. The one-sided pictures. The most difficult thing about balcony gardening is that all the light comes from one direction. So the plants lean towards it, away from the house, and the container ends up lop-sided with the plants trailing over the balcony railings away from you. All you get to see from your living room are the backs of flowers and leaves leaning away. And tall plants end up looking like the leaning tower of Pisa. Smaller containers can be turned regularly, but that doesn't help with the large fixed ones. Look at the one in the photo for example - super. But what's it like from the other side?
4. The perfect colour schemes, achieved only by going out to buy an exact variety of a plant. Phyllitis scolopendrium "Cristatum" - whaaat ? Here I'd be lucky if I could get the plant, let alone a specific variety. I have sometimes thought of going to our local garden centre with a list and saying "I'm looking for these." I suspect the reply would be the Italian equivalent of "What about some nice pelargoniums, luv?"
OK, OK - I'm just envious because my containers never look as good as the ones in the books. But that doesn't stop me yanking them out every spring, poring over the pictures and trying again. And buying more. Maybe this year ...