Never pluck flowers during evening. Always express gratitude to plant while plucking flowers. Traditionally a mantra is also chanted while plucking the flower. Don’t offer flowers which are picked up from ground. Offer fresh flowers and not stale flowers. That said and don’t offer flower which is not bloomed up fully or the flower whose petals have withered away.
Flower buds are not offered. But buds of champa and lotus are two exceptions. Don’t steal the flowers or beg for them. Don’t offer flowers which are washed with water. However you can sprinkle some water on them. Don’t offer flowers which have diseases or infested with insects or damaged. Don’t pluck tulsi from plant on the eve of sankranti, dwadashi (12th), amavashya (no moon), purnima (full moon), Sunday and during evening hours.
Lotus stays fresh for around 5 days. If you are not getting new leaves of bilva patra, then the leaves which are already offered to god may be reused after washing. Lord Shiva never liked champa (champaka) and ketaki or kewda (fragrant screw pine) because it was cursed by lord Shiva along with Lord Brahma for lying. Lord Ganesha never liked ketaki flower and tulsi.