Most flowers and their plant parts are good compost ingredients. Compost is an organic soil enrichment that combines greens -- such as newly cut flowers, other fresh yard waste and kitchen scraps rich in nitrogen -- and browns that are high in carbon, including dried flowers. Both garden flowers and cut, commercial flowers are beneficial in compost. However, there are some kinds of floral waste to avoid, because they are difficult to decompose or include compounds that may add poisonous residues to soil.
Don't add thorny rose stems, bulbs, any flowers treated with toxic herbicides and certain poisonous plants to compost. Exceedingly slow decomposition is a problem for thorny stems as well as bulbs, corms and tubers, which can take up to two years to rot. Investigate thoroughly before adding any plants with poisonous parts to compost. Some, such as the flowers and foliage of angel's trumpet plant, are rendered harmless through composting. However, hemlocks are extremely poisonous and leave deadly toxins in soil even when decayed. Always wear garden gloves to protect yourself from toxins. Also, don't pour cut-flower water containing preservatives into compost.