Annuals in your perennial garden are something to think about! Annuals give you season long color, easy propogation, they're cost efficient, also provide first season interest.
If you're just starting a perennial garden annuals are a great choice to fill in gaps. I remember my first garden. I bought a couple of plants thinking "This is gonna be great!" A few weeks later I realised this was not true. I needed a whole lot more plants also they cost a bunch! I opted for some annual seeds also I had a really pretty garden by the seasons end. Annuals can certainly help to fill in a garden while you wait for perennials to mature.
Even an established perennial garden may have spots where little is blooming at one time or another. Annuals are a great way to fill those gaps also keep color in your garden. They bloom from summer to fall also with a bit of dead heading you can continue to coax them back.
Annuals are simple to start from seed which makes them very cost efficient. You can certainly purchase annual bedding plants from a local garden center, however mail order or seeds at the garden center offer you a much wider variety than your standard petunia, pansy or snapdragon.
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For more information on perennial gardens also gardening in general see http://www.agardenwalk.com