Answer: Unfortunately, without knowing if the location is in full sun or shade it is impossible to make a definite suggestion. As you may be aware, most perennials bloom for only a limited time each year so you will not be able to achieve that "bedding" look using them. A mass planting of any one perennial in bloom can be stunning however, for the time it is in bloom. Since you are looking at such a large space you could conceivably plant several large blocks, each of a different plant, and extend the bloom display that way. This will also be easier to maintain than an interminlged planting would be. In either case, there are many possible choices and the selection should be made based on both personal preference and on the growing conditions including soil type, sun exposure and type of maintenance available; perennials do require ongoing care even though they may not require replanting every year.
Since you are just beginning to think about this I would suggest you look at a few books about perennials and perhaps consult with some gardeners in your area to find out which plants seem to do especially well. Two possible choices to look for would
be first as a thorough introduction to perennials "Perennials
for Dummies" by Marcia Tatroe and the Editors of the
National Gardening Association ISBN 0-7645-5030-6;
and then as you gain experience the very detailed
"Perennials for American Gardens" by Ruth Rogers Clausen
and Nicolas H. Ekstrom ISBN 0-394-55740-9.
You might also run some soil tests and use the results to assist you in preparing the planting area. For best results, you will need to prepare the soil thoroughly by adding copious amounts of organic matter and any other amendments as indicated by the results of the tests. Your UMASS extension can help you with the tests and interpreting the results, they may also be able to make some plant suggestions.
In terms of a ballpark figure, possibly the least expensive method is to grow the plants yourself from seed, the next would be to purchase smaller sized plants and the last (more of an instant effect) would be to purchase larger plants at the outset. I would say a rule of thumb could be devised by knowing the approximate recommended spacing of the plants you want to use and calculate how many you will need based on the square footage of the area. You may find that a local nursery will allow you a price break on a large order of all one plant, especially if you talk to them about it now while they are placing their wholesale orders (or propagating in house).
Enjoy your project!
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