If you want to make some serious changes in an existing garden, the best way to start is by analyzing what's already there. Take copious pictures and notes over an entire garden year. If you want to look out a window in the winter and see something other than dried plant stubble, that means your garden year encompasses all twelve months. Make notes of what you like and don't like about the garden: too crowded, not a balanced bloom or color schedule, great in spring, weak in summer, love the delphiniums, phlox badly mildews, asters flopped, etc. Include observations you've made in other gardens of what you do and don't like. If you don't write these things down and take pictures, you won't remember them all. Make sure to include some facts about your garden needs. If what you want is vegetables but all your space and time is devoted to flowers, you're not going to be a happy gardener. There's no reason you can't have both in the same garden.
If you're really unsure about the whole process and lack trust in your ability to make useful changes, start slowly. A few successful decisions breeds confidence, and you can do a lot of experimenting with all those pictures. Find a versatile copy machine and start cutting and pasting. Blow your pictures up to a size you can work with and edit out a few plants. If you want to add some major structural feature like an arch or trellis, or a small tree or shrub, draw in a facsimile of what you're adding. Don't worry about whether you can draw, go for general shape and scale.
Compare what you like about the garden with what you don't like and try to discover why. For example, many people love their gardens in the spring and become more dissatisfied as the season progresses. With pictures to compare, you might notice that in the spring there's space between plants, their shapes are clearly defined and individualized. Emerging colors are crisp and clear; they have less competition in the landscape. The difference between the greens is more obvious; they contrast with the dark soil, not just each other. We also tend to have more in bloom in the spring. Desperate to get outside and eager for color after winter grays, we're extremely susceptible to buying binges. What are we drawn to buy? Plants that are in bloom. With only so much available space in a garden, an indulgence of early blooming plants means less space for the rest of the season. Buying plants solely because of their flowers also exacerbates other design problems. After all, perennials only flower for a few days or weeks within a season that extends from April through October.
Continue to compare and analyze your garden photos. Chances are the further into the season, the less you see the edges of individual plants. All those nice spring shapes have become an amorphous mass. Make a list of the plants in your garden and reclassify them by plant shape. Multi-stemmed and columnar: Phlox paniculata, aster, delphinium, helenium, heliopsis, monarda, Veronica spicata, aconitum, etc. Round or mound shaped: Lady's Mantle, hosta, euphorbia, peony, heuchera. Spiky: iris, liatris, eremurus, yucca. Further separate them into short, medium, and tall, and then look to see how many you have next to each other. In your spring photo those plant combinations may look fine. Not only can you identify their individual shapes, but the heights and widths vary from the front of the garden to the back, not always with the tallest in back. Because plants emerge from dormancy and grow at different rates, nature has temporarily skewed your careful placement. And doesn't it look nice to have that regimen of short to tall broken up? By July, most everything has reached its mature size and a dense hedge-like look can dominate. Breaking up the density can make a huge visual difference.
Take a marker and trace the top contours of plants in your seasonal photos. You'll probably find a greater variation in the earlier photos, and you may never trace a contour down to ground level in any of the later photos. In fact you may find it difficult to find distinctive contours at all and those that you do find might not have much variation. As a first resort, thin existing plants out. Often, several plants are grouped together where one mature plant would fill the space. It's better to err on the "less is better" side--financially, visually and physically. Adding plants rarely incurs the hassle or trauma that removing them does.
If you are bothered by the empty space surrounding an immature plant, fill the space with an annual or vegetable. Want extra basil for pesto? Tuck some into your perennial border. Better yet, choose one of the more decorative varieties with ruffled leaves, purple foliage, or a perfect little globe shape. Rearrange plants so that their different shapes, textures, and foliage colors emphasize distinctions between individual plants. This helps prevent that dense hedge-like appearance. Such plant combinations are the basis of garden design.
Another tip, pay attention to the potential size of plants. Seek out mature specimens in other gardens. Reading that a plant will grow to five feet high by three feet wide makes less of an impression than seeing one that size in situ. This is especially true with trees and shrubs (think overgrown foundation planting). It's hard to keep a plant from becoming the size nature intends, but it is possible to choose a more suitably sized plant, despite all the bad examples you see.
Notice the differences between a mature single plant and several of the same plants massed together. Often the contours are extremely different. One large, full-grown hosta looks very different from three planted together and grown into each other. The same is true with Lady's-Mantle, cimicifuga, baptisia, euphorbias, and many others. It helps to know which look you want or like. Conversely, with some plants it makes no difference. Most aggressive plants fall into this category: monarda, physostegia, yarrow. There is little need to buy three plants if, by the season's end, one will have filled the space.
Which brings up another issue: space and spacing. I can guarantee you will plant things too close together Most professionals have grown hundreds of plants to maturity, moved as many, designed and planted many gardens and continue to overplant. Most books and garden plans encourage overplanting ,though they don't label it as such. Everyone desires instant beauty, instant gratification. It's hard to love the look of a perennial garden with small plants surrounded by large empty spaces, even harder to pay someone to create it. Knowing this, you'll probably still overplant, but with the understanding that it's a human failing not a desired goal.
So what do you do if you find your garden has too many plants overall and too many similar shapes? It's easy to say--take some plants out and move others around--harder to tell you how to and which ones. Start by making a list of the plants you could do without. You won't miss that mildewy phlox, aggressive artemisia or a few dozen Forget-me-nots. Remove completely or reduce the number of such plants.
Analyze the seasonal attributes of your plants; what are their positive and negative characteristics? Take for example oriental poppies. Everyone loves the luscious flowers and feathery foliage early in the season. But the fact is the foliage flops, eventually browns, and goes dormant for a good part of the summer. It doesn't make sense to have a huge clump looking bad for so long. Put these disadvantages to good use by placing small clumps next to plants that start off slowly and would welcome the space the poppies leave mid-season. On the other hand, herbaceous peonies have great value over the entire season. Those vibrant red stalks that arise and uncurl in early spring, beautiful flowers (if a bit floppy) glossy green foliage that turns into great fall color, and a shrubby rounded shape that holds its distinctive contour all season The more plants you have with multiple positive attributes, the better your garden will look.
Make some changes then reevaluate the scene. Take some more pictures and compare them with early versions of the same areas. You should begin to see and better understand the importance of plant shape as a design element and how much difference the space between plants (even just a few inches) can make. Allowing adequate space for a plant to develop its individual characteristics, and choosing neighbors to compliment or contrast these characteristics, is the essence of garden design. But don't be overwhelmed by thinking you have to have perfect combinations with every plant. Start with a few good ones to sharpen your eye and then branch out. This type of fine tuning can be a life process.
Don't forget to consider adding a few strong architectural elements. Imagine adding a dwarf crab that has a lollipop or umbrella shape to your garden. A conifer, conical or pyramidal and green all year. A tree peony for a large, relatively open, rounded shape. A great clump of tall grass. Maybe a birdbath or piece of statuary. Each has a its own strong shape and texture. Which one would add the most interest to your garden? Experiment with placement by trying several different locations. Notice how the shape affects the contours. If your choice is too cumbersome to move around, sketch it into your photos with a marker. Once you've decided on the perfect spot, make sure you don't weaken the architectural impact. If you've chosen the umbrella shape of a single-trunked dwarf crabapple, keep surrounding plants low. If you plunk a large clump of tall phlox next to it, or obscure the trunk with a dense underplanting, you lose the distinctive shape the tree adds to the garden.
If you break these changes up into manageable bites and continually reassess the results, the prospect of renovating your garden won't be so overwhelming. It's not a bad idea to spend a year simply cleaning up what already exists. Clarifying what you have can highlight the positives as well as the negatives. You might begin to see your garden from a different perspective, be less apt to throw the baby out with the bath water, and maybe even find that a few simple changes are all that's needed.
Whatever you do, always keep in mind what your garden goals are, and check back as you plot strategy to make sure you're not obliterating them. Remember, your garden is YOUR garden. It doesn't need to look like someone else's. It doesn't need to have great contours, or varying textures and foliage, if that's not important to you. The health and well-being associated with gardening gets obliterated if your experience becomes stressful and traumatic. So be creative, be whimsical, but above all enjoy!