You will find that cats and dogs are far more clever than we think and know most plants to avoid as they have a more sensitive nose than we have, however it is always safer to know what plants you are growing should anything go wrong and the animal takes ill.
Growing Catmint (nepeta) is the best idea for your cat as this is also a tonic for the cat, they love it, I grow several in my garden as one single plant will soon be devoured by the cat so an extra few helps save the plant from total distruction as there is more time to regrow if there are more than one, they are also very beautiful low growing feathery, ground covering plants. I am not sure IF a Passion flower will survive your cold winters outdoors in the Netherlands, I think you get a really cold night frost too and the foliage and roots in a pot would possibly be killed by these conditions. I would be more inclined to search for plants that grow all year outside in your climate, but grow these in Larger pots, you could go along to your local Library/bookstore to look at books on growing plants in a small garden or container garden that would be able to endure your cold winter conditions, even if it means you would have to offer these plants some winter protection with things like horticultural garden fleece wrapped around the plants for the winter and remove this once you see new growth appearing come spring. Plants are expensive as are good quality pots, so it would be a shame if you spent a lot of money and then the plants never survived the cold winters, also some pots will crack in really cold spells as the soil can freeze and expand inside the pots killing the roots, I would hate you to be so disappointed, so maybe do some further research on pots and plants to survive your winters outdoors. Good Luck. WeeNel.
