Proximity is one of the most difficult things I deal with in the landscape design business. The reason this is so difficult is because plants do not always grow to the exact heights and widths that are published in a book or even a catalog written by a grower. Depending on where an author or grower lives can have a great effect on how large plants may get in that particular area opposed to mine. Two other factors that can contribute to differences in size are light and moisture conditions.
There are good books on plants and there are bad books on plants. One way to determine a books worth is whether it has zone ratings for the plants listed. If not its recycle bin material more times than not for me. Two books I rely on are Michael Dirr’s Manual of Woody Landscape Plants (fifth edition) and The American Horiculture Society A to Z Encyclopedia of Garden Plants. These two books can help you put into perspective how large a plant may get. Look for books that have lots of Zone 3-7 plants in them. These are most applicable to our area which is Zone 5. If a book is by an author in Tennessee and you see lots of zone 6-9 plants you will not gain much useful info if gardening in the Midwest.
At one time I had an assistant designer that was formally trained and she told me when drawing a design she would only draw a plant to two thirds of its mature size listed in the literature. This rule has its place as well. Many times it is important to take into consideration how long you will be living at the residence you are landscaping. Do not fool yourself. The people that will follow you will probably not like what you like and may very well rip everything out. It is sad but true. So that being said design for yourself in most cases unless you are landscaping to resell your house.
Knowing what we know now, to design with proximity in mind generally requires some bookwork. Even after twenty plus years in the business sometimes I still look plants up when drawing a design. There is always more to learn about plants.
It is important to take the shape of plants into consideration when considering proximity. A eight foot wide Japanese Maple ‘Fireglow’ which is vase shaped can go reasonably close to a “Sester’s Dwarf” Blue Spruce which is pyramidal because they fit together like puzzle pieces. If drawing a design with circles at ¼” scale as I do your Fireglow would overlap your “Sester’s Dwarf” by 2’ or so.
There is a lot of gray area in regards to proximity, eighty percent or so. The black and white part to me personally is the 2/3 rule mentioned earlier. That being said this is more an artistic endeavor than a one of hard science.
Consider proximity when designing your landscape. Taking it into consideration can put many extra years on a design.