Multiple species of evergreen trees are colloquially referred to as cedar trees in the United States. Two native cedar tree species are Juniperus virginiana, often called eastern red cedar, and Thuja occidentalis, sometimes called western cedar. The famed cedar of Lebanon, Atlas cedar and deodar cedars belong to the genus Cedrus, and are what botanists regard as the true cedars. If garden plants falter around eastern red cedar trees, it may be caused by soil chemicals or the more aggressive growing abilities of the cedar tree.
When plants secrete toxins into the environment to kill nearby competing plants, it's known as allelopathy. The most famous American native tree that is allelopathic is the black walnut tree. However, many juniper shrubs and trees, including eastern red cedar, secret oils and other chemicals that react with the soil to harm other plants. This gives any small red cedar trees an advantage in growing since there are few or no other plants competing for sunlight or water and nutrients in the soil. All junipers tend to dissuade the growth of grasses nearby.
Eastern red cedar is a pioneering plant species, meaning it is among the first trees to sprout and vegetate a cleared area across its native range, which is roughly the entire nation east of the Rocky Mountains. Easter red cedars grows well in a wide range of soil pH, types and fertility. The tree is quite drought tolerant, but has an extensive, fibrous matrix of roots that will thoroughly absorb soil moisture to sustain the tree. The roots may linger at the soil surface, making it difficult for other plants to sprout and send down roots. The soil remains rather dry because of the cedar tree's root system, making it less hospitable for other plants to survive. Any large cedar tree, regardless of species, can create a soil environment too dry for other garden plants to prosper.
With potential allelopathy and dry soil already occurring in close proximity to an eastern red cedar, the shade cast onto nearby plants can limit growth. Depending on the other garden plant species planted near the cedar tree, each needs varying amounts of sunlight in order to make food in photosynthesis. The cedar tree may block the sun and not provide adequate light for other plants to prosper. Add in a dry soil, and many plants simply become too weak and succumb to disease, insect pests or just slowly degrade and die.
Not all plants are affected by the chemical compounds that occur in close proximity to eastern red cedars, but there aren't many lists available that mention which common garden plants are resistant. Therefore, improve the soil conditions near the eastern red cedar where you want other garden plants to grow. Adding lots of organic matter, such as compost and rotted manure, helps diminish the concentration of allelopathic compounds and the soil microorganisms may more rapidly break down the chemicals. Irrigate to keep the garden bed evenly moist to benefit the garden plants. Another option is to create a raised bed with fresh soil atop the ground where no cedar tree roots or fallen needle debris accumulate.