Answer: That depends largely on two things: how much of a hurry you're in, and how much you're willing to invest in the project. Quicker results cost more. If you critically need screening soon, and cost is a lesser consideration than time, you could buy the largest size available of the species you want, and plant one every three to four feet. By doing this in spring, in the summer of the year following planting you will have pretty good screening, and excellent screening by the summer after that.
If you have a longer time horizon and/or have a smaller planting budget to work with, you could buy smaller plants, and increase the spacing up to eight feet between plants. This will still give excellent intermittent screening by the third year, but may take up to five or six years for the clumps to fill in the gaps and give continuous screening.
If you've got a really small budget, a long time horizon, or a really long fence line to cover, you can even plant out just a few plants, take good care of them, and after a few years divide them yourself to plant along the rest of the fence line.
Hope this information is helpful!
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