Thank you for your question.

Arborbitae, botanical name is Thuja; family Cupressaceae, constitutes a major group of small to medium (some large) size evergreens used in the landscape. Many cultivars of different form, size, and with varied foliage color are available. Five species in cultivation are native in North America and Eastern Asia. Thuja orientalis (which is more correctly called Platycladus orientalis) is the least hardy of the species and is used extensively in the Southeast, Southwest and West. Some cultivars of T. occidentalis are of good quality, however, many types have a tendency to discolor in the winter, with center foliage browning in the fall.

Arborvitaes are not considered to be of the highest quality because of winter discoloration, loss of foliage, and a thin and ratty appearance with age. Because of this, many types tend to decrease rather than increase in value, especially in congested planting sites. Thuja occidentals cultivars ‘Emerald’ (‘Smaragd’), ‘Nigra’, and ‘Techny’ are very valuable as they maintain good green foliage color in all seasons. The hybrid ‘Green Giant’ has become a popular choice in recent years.

Leaf browning and shedding –inner leafdrop in the fall may also be due to physiological diseases. Although in general these plants exhibit few serious insect and disease problems. But: http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/needlecasts/pestalotiopsis-tip.aspx

Culture –

Arborvitaes should be planted in fertile, moist, well-drained sols, although in the wild the species may be found on wet and dry soils (maximum growth is not realized on these sites). They are easily transplanted balled-and-burlapped or as container grown plants about any time of year. They perform best in full sun, although light shade is acceptable. In heavy shade plants become loose, open, and lose their dense constitution. Thuja plicata appears more shade tolerant than T. occidentalis or T. orientalis. Pruning can be accomplished prior to growth in the spring and in summer.  Usually extensive pruning is not necessary.

Hope this is helpful.