What is the difference between an annual, biennial and perennial?
An annual is a plant that completes its life cycle within one growing season - this means that the plant germinates, flowers, produces seed and then dies within the first year. Often annuals will "re-seed" freely however, meaning that as they come to the end of their cycle, they drop their seed and these then germinate the following season to provide you with new plants as the cycle starts all over again. For more "tender" annuals, if you want plants the following year it is often worth collecting the seed and re-sowing once the soil warms up the following year and there is no signs of looming frosts.
A biennial, completes its life cycle over two years. This usually means that the plant germinates, grows and establishes in the first year and then flowers, seeds then dies the following year. Again, these plants will often re-seed freely in their second year but to be sure of creating some more plants it is a good idea to collect seed and store until the next growing season begins.
A perennial is a plant that returns year after year, although strictly speaking it is any plant that survives for three years or more. A herbaceous perennial is one which dies down almost completely over winter and then re-grows the following season -so if you have one of these, don't think it has simply died over winter and then go digging it up for the compost bin - it will come back! It's worth noting that with some herbaceous perennials such as delphiniums. the new shoots that do emerge in spring can be attractive to slugs and may need some protection. If you're expecting a plant to return but don't catch any new growth, look out for signs of these pests and act accordingly - perhaps with some organic pellets or protective barrier.