I would definitely regard Aster x frickartii ‘Monch’ as one of my ‘must have’ plants. It flowers right through until the end of October. Its lavender blue flowers may not be particularly spectacular, but, it gives great value for money. It works well with so many other subjects, and is so obliging. It’s self supporting and tolerates a bit of shade with none of the mildew problems of so many of the herbaceous asters And the bees make a bee line to it.