The use of bright colours in the planting is key: "shock and awe" is the way to get a big bang. Daffodils and tulips return year after year. Canna lilies are very striking with their pink, yellow and orange brightly coloured flowers. Primroses (this is England after all) make nice spots of colour, in blinding colours and have a long flowering period. Incongruity: plant something large and outrageous which will really stand-out. Sunflowers grow to 15 feet in dry soil. Christmas trees are hardy and look green all year. Fragrance: lavender and sage have lovely purple flowers and are sweet smelling. Mock orange is a tall, fragrant shrub that survives in poor soil. And then there are seed bombs .
Scattering seeds is the easiest way to guerrilla garden—just throw them in and some will live and flourish, others won't. You can even do it while driving. However, they do need to land in favourable growing conditions. Throwing dry seeds onto a mound of garbage won't work. The answer: seed bombs. Usually they are made of seeds, compost and a bit of water in a biodegradable container. One dedicated "cell" of guerrillas stuffed the mix into empty egg shells. These self-contained little missiles which are packaged like grenades can be fired (thrown, really) into inaccessible places.
Dealing with vandals: If you cannot face battling the idiots who vandalise the gardens, then make yours less showy. It is the dramatic and exotic plants that draw attention. Or plant them in large clumps so that the loss of one doesn't stand out so clearly. :: On Guerrilla Gardening: A Handbook for Gardening Without Boundaries Via :: Guardian