A hedge makes a good boundary marker or privacy screen. Formal hedges need annual trimming to keep tidy. The hedge would be one straight line of shrubs. If starting small (as a rooted cutting or bareroot plants) you would plant them every 2 ft. Think about the hedge in 3 to 5 years, do you plant it close to property line so it grows through the fence? And will your neighbor keep that side trimmed? Or do you plant it 3ft. inside your property boundary so that you trim both sides?
There are many shrubs that grow quickly and make great hedges. Some popular ones are forsythia, spirea, junipers, hollies, arborvitae. There are many varieties of these so read the labels for height
Forsythia is inexpensive, and easily propagated. It can be easily clipped and pruned, provides early spring (yellow) flowers, and has some reddish fall color. Being a spring flowering shrub, the flower buds form in the fall. So clip this one after flowering in early summer, not in the fall.
Spirea or Bridal Wreath is an old favorite. There are several varieties available now with pink or white flowers, and in different sizes, so check the tag for expected height. If they are expected to grow 6ft tall and 6ft wide, then you could plant them 6 ft feet apart; but for a formal clipped hedge you would want to plant them closer, say 3 to 4 feet apart.
(above: Dwarf Spirea with summer flowers)
So while forsythia and spirea loose there leaves in the winter, there are many popular evergreens used for hedges as well. Junipers are available in many sizes and shapes. They may be groundcovers, shrubs, or trees. The ‘Hetz’ juniper is a good choice for a formal hedge, as it can be sheared to keep in bounds and grows in a columnar fashion. Foliage may be greenish or grayish, and is prickly.
Hollies are popular as an evergreen hedge. Again there are many varieties available; they may be upright, spreading, small trees or dwarf. The Chinese varieties mostly have prickly leaves. Also, if you want red berries in the winter you need to make sure you get a male plant to pollenate the female plants. The Japanese hollies do not have prickly leaves and its berries are black.
(Above: Japanese holly)
Arborvitae is a popular tall evergreen hedge. The upright varieties are small trees that grow in a conical shape. They can also be pruned or sheared into a formal hedge.
There are many more kinds of plants that can be used for hedges. When visiting the nursery read the labels for size, habit of growth and popular uses. A good label is invaluable. If you need a lot of one type of plant, talk to your nurseryman. He may be able to order a quantity in the size you want.
(P.S. Do not choose an invasive plant. The label is not going to tell you that, so you must do a little research for this information. Most states publish a list of invasive plants which is easy to find on the internet)