When purchasing a tree from a nursery, the least expensive type is:

The least expensive type of tree when purchasing from a nursery is typically bareroot. This is because bareroot trees are sold without soil surrounding their roots, which reduces shipping and handling costs. They are usually available during the tree's dormant season, making them easier and less expensive to cultivate and transport compared to other types.

Balled and burlapped trees involve additional preparation where the roots are wrapped in burlap and soil is contained, increasing both their weight and the cost. Containerized trees, while easy to handle and plant, also carry additional costs due to the pot and soil they are shipped in. Potted trees are similar in that they require pots and soil, which adds to their overall price as well.

Therefore, bareroot trees present the most economical option for buyers looking to reduce costs while purchasing trees for landscaping projects.