A man called my garden talk show last weekend, and he presented a question I don't remember answering in 35 years of doing radio. Since those years encompass what I've guessed to be a third of a million questions, I told him that I found his inquiry very interesting."Neil, I'm just wondering what your thinking process is in choosing the best plants for a landscape." As we spoke, I found out that he was asking about selecting perennials, so I shifted my reasoning in that direction.Fall is a great season for getting perennial gardens started, so his timing couldn't have been better. He said his neighborhood group was trying to establish consistency of appearance, and he said that they needed to be fairly low-maintenance plantings.I'm sure every landscape designer and nurseryman has his or her own criteria and a sequence by which they're put in place. I don't pretend that I have the only right answer. But, here are the mind-steps I take when I'm choosing perennial plants I'm going to use in my landscape.First and foremost, the plant has to be adapted to the place where I'm going to be using it. There's no point in even considering a plant if it isn't going to be happy with its chance to grow in my gardens. It has to be able to withstand the heat of a long Texas summer, and it has to be winter-hardy as well. I can modify the soils, and I can even adjust to plant it in sun, part-sun or shade, but any plant that can't hold up to our weather isn't going to get picked up and put in my cart.And, while we're on that "temperature" thing, I still consider my garden to be in USDA Hardiness Zone 7. I'm not forgetting that the latest update moved the somewhat more tropical Zone 8 several counties farther north, into southern Oklahoma, and I know that Tarrant County has been listed as Zone 8 for years. But, Zone 8 calls for winter temperature minimums to be between 10 and 20 degrees Fahrenheit.I've seen it much colder than that in the Metroplex in the decades that I've lived here. I just don't like surprises, and I don't want plants to die just because we have one very cold winter somewhere along the line. Oh, I'll try a few Zone 8 plants, but I do so in extreme moderation. I've had my heart broken just a few times too many.Second consideration is a perennial's color. I try to have a color scheme in my gardens, so it's important that all of my players work well together. The last thing that I want is one bright yellow flower popping out of a bed of otherwise lavender, pale pink and white blossoms.Tying right in with the plant's color is the timing of its bloom. Most perennials flower for two or three weeks, then they shut down for another entire year. You have to have a constant sequence of blooms month after month to keep the garden interesting at all seasons.The best way to incorporate colors and bloom times is to have a 52-week calendar. Determine what your spring color scheme might be, then one for the summer and a third for the fall. It might be cheerful pastels for the spring, cooling blues and purples in summer and rich fall hues in autumn. Then, accumulate all the plants that bloom (a) in those colors (b) at those seasons.This is about the time at which I'd begin to think about each plant's mature size, growth form and texture. I'll have to consider where in the bed each will be planted -- front, middle or back -- and that will be determined by its mature size. I'd like to have an attractive blend of large-leafed, boldly textured plants growing alongside plants with softer, finer-textured appearances.The secret to successful perennial plantings is to have something colorful at all seasons of the year. Those little "pockets" of color will migrate around as the weeks progress. Tying it all together, I include several evergreen shrubs as the "bones" of the garden.Just because a garden features primarily perennials, that doesn't mean that you can't use annuals. Since they have showy flowers and foliage for months at a time, they're great sources of continuity in your plantings. Fill the small spaces with dependable annuals.Most perennials perform best in full or nearly full sun. They grow to full potential in rich, highly organic garden soil. Use a glyphosate weedkiller now to eliminate all the existing grass and weeds where you want your new bed to be.Wait 10 to 15 days, then rototill and rake to remove all the roots and other debris. Incorporate 5 or 6 inches of organic matter, including sphagnum peat moss, compost, rotted manure and pine bark mulch, plus 1 inch of expanded shale, then rototill again to a depth of 12 to 15 inches.That done, you're on your way to a beautiful garden for the ages.Neil Sperry publishes "Gardens" magazine and hosts "Texas Gardening" from 8 to 11 a.m. Sundays on WBAP AM/FM. Reach him during those hours at 800-288-9227 or 214-787-1820.