Here are six examples of plants to cut back in August for an extra flush of flowers. 6 plants you need to cut back in August for an extra boost of flowersHardy GeraniumsHardy Geraniums, known for their resilience and long-lasting blooms, can start to look messy late in the season. "Lady’s mantle will often respond well to a trim, especially if you remove spent flower stems and any tired growth." Jane said: "Keep removing faded flower stems, cutting back to a healthy sideshoot rather than trimming the whole plant indiscriminately. Jane said: "One important caveat is that plants need water to regrow.