Answer: Most plants do best if planted in early spring or early fall, mainly since the summer can be hot and dry and cause the new plants moisture stress. If they are container grown, however, and if your soil is well prepared, and if you water them faithfully and keep them mulched they should do fine.
Clay soil usually benefits from the addition of copious amounts of organic matter such as compost, rotted leaves, aged stable manure and bedding and so on. You should also run some basic soil tests to see what your soil is (and is not) lacking. Your County Extension should be able to help you with the tests and interpreting the results. They may also have some recommendations for plants that do particularly well in your area.
Since it sounds like you may be new to gardening and landscaping, I would suggest you read a book or two about it before trying to make specific plant selections. One I particularly like is "Gardening for Dummies" by Michael MacCaskey, ISBN 1-56884-644-4, Dummy Press. In it you will find guidelines for evaluating your site, preparing the soil, selecting plants, and maintaining them.
Enjoy your new plantings!
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