Answer: In most instances it is not necessary to add pebbles to the bottom of the pot, particularly if your pot has drainage hole(s) as it should. One of the reasons for this is that the plants usually need every inch of space for soil! Do keep in mind while planning your container garden that the larger the pot the better in terms of root area and moisture holding capacity. Also check each plant's soil preferences, since some (especially among the trees and shrubs) are particularly sensitive to either moisture levels or pH. Finally, think about how you will be protecting the plants during the winter if you plan to keep them after next summer.
Since you are somewhat new to container gardening, you might want to take a look at a book or two about the special considerations for this type of gardening. One I particularly like is "Container Gardening for Dummies" by Bill Marken, ISBN 0-7645-5057-8. In it you will find in depth information about recommended pot sizes, soil mixes, watering techniques and suggested plants to try -- from flowers to veggies to trees.
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