Look at the August 2011 What is Blooming in your Garden Thread.

FWIW - many perennials will not bloom the first year. This is particularly true if you start from seed. Even mature plants from nurseries - some undergo transplant stress and do not bloom, and some were forced to bloom early, and thus will not bloom during their proper bloom time (think all those lillies that were forced to bloom for easter). So buy from a reputable nursery, explain what your goal is and ask if the plants will bloom this year.

Here is a link that might be useful: August 2011 What is Bloomning in Your Garden

Many annuals might work well for an August display - just not the early spring ones - various poppies, larkspur maybe, could be all gone by late August. Black eyed susans will definitely be in bloom, even if you do a great job eradicating them - I know this from personal experience.

Can you tell us how much sun you have? SOmething like lavendar could be great - it usually blooms well the first year, if you start with small plants; some varieties (Lady, plus the frilly 'Spanish' types) are often treated as annuals.

That's also when Russian Sage is in bloom, if you have a lot of sun that might be an option - it generally blooms well the first year.

As diggingthedirt suggested, knowing a bit about what your yard's conditions are as well as your gardening preference (symmetrical and tidy, wildly blooming but disorganized, ...?) will help you get suggestions. A photo will also help us envision what might look nice. Along with flowers also consider variegated or colorful (gold, maroon, etc.) foliage and contrasting foliage texture (ferny vs broad vs. long & slender vs. cut-leaves), as often it is decorative for longer than just flowers.

Below I've linked a thread asking about an early VT wedding which may have some ideas for you, including some specific suggestions for annuals that will bloom for all of August under normal conditions.

If you go to the bottom of the main New England forum page there is a search box. Type in wedding and look at the various threads, even if they are for the wrong time of year. Several of them discuss the potential pitfalls of this type of a plan. They will help you evaluate whether you really want to try this for yourself on such short notice. In general if you want to use perennials many would want to be started the previous year. You can use annuals if you choose varieties that will still be blooming in August and as Digging suggested, there are some perennials that will bloom the first year.

If you end up deciding to do this, have a backup plan in case a dog digs up your beds, a hurricane arrives in August and flattens everything, or the weather is odd and nothing ends up blooming at its expected time. It might include scoping out nurseries ahead of time and then buying well grown pots of annuals and putting them in elegant planters or obtaining reallu large vases in which to place tall cut flowers.

Paniculata Hydrangeas (shrubs) will probably be blooming then, but you would have to choose one of the smaller selections like 'Little Lamb' since your garden is small, get the largest potted plant you can find and hope that it is happy enough with its new site that it blooms this August. There may be some clematis that would bloom in your area in August, but again, you'd need to buy the largest plant you could find and plant it with as little disturbance as possible so that is more likely to bloom.

Regardless of what you plant, plan to spend time weeding and watering on a regular basis this summer. Using pretty mulch (like cocoa hulls if you don't have a dog) will help keep moisture in your beds and will discourage some of the potential weeds, but both will need attention if the garden is to look its best.

I am not trying to discourage you, but I would also hate for you to be disappointed that things don't look as you envision on this special day. Let us know what you decide to do. We love seeing photos . . .

Here is a link that might be useful: Early August VT wedding thread

This is perfect timing to plant the taller growing types of dahlias which will be blooming heavily in late August. Actually you can find a nice assortment of packaged roots in the box stores right now. Check packages carefully for signs of emerging spouts. As you know, dahlia flowers come in all sizes and colors. With a bit of searching among the roots in box stores and nurseries you should be able to locate three plants that would give you a pleasing blend of colors in your limited space. Keep well fed and staked.