There are a number of plants that will deter flies. These are best grown around exterior doorways. Good examples are basil, tansy, rue, mint, elder, lavender and pennyroyal. You will not stop all flies entering your house so it is wise to remember that light attracts them. If you have lights on and open windows then the flies will enter. Flies do not like smoke so you could burn incense sticks or candles containing citronella. There are also carnivorous plants like nepenthes, also known as the pitcher plant or monkey-cup, which are ideal for a greenhouse or conservatory.