It’s time to get to the root of what to look for when buying trees and shrubs.

Shoppers spend a lot of time searching for the right plant. They want to be sure the shrub or tree is the right variety for their yards or, if they are a bit more adventurous, if it will do the task they’ve set for it, no matter what the variety: Do they want it to block the view of the neighbor’s shed, for instance, provide food and habitat for insects and birds, or just look pretty? They’ll probably read the label to see how big it will get and whether it needs sun or shade.

Then they will look at the plant carefully, to see if the leaves are healthy and the branches unbroken and growing in a satisfactory direction. Most buyers stop there.

What they should do is check out the roots, Jeff O’Donal told an online gathering of horticulture-industry professionals co-sponsored by Coastal Maine Botanical Gardens and the Maine Landscape and Nursery Association. Buyers should know how much of the plant’s original root system they are getting and how healthy those roots are.

If you’re buying from local nurseries, the options for shrubs and trees are either potted plants or balled-and-burlapped plants.

When you plant a tree or shrub from a pot into your own garden, be sure to spread out the roots so more of them come into contact with the soil. Shutterstock/PA

Pots have come a long way since O’Donal worked as a youth at what was then his father’s Gorham nursery; O’Donal now runs the nursery, as well as McSherry’s Nursery in Conway, New Hampshire.

“We used to take a King Cole potato chip can when we wanted to put plants in a container,” O’Donal said. “There were no plastic containers available. And we planted in loam.”

Specialized container mixes that are much better for potted plants were way in the future, he said.

Container plants used to mean small plants, and for many home gardeners, that is still the way to go.

“If you enjoy watching things grow, you should buy a one-gallon plant,” O’Donal said. It’s a lot less expensive, the transplant shock will be less, and watching the plant’s development will be educational. I like to think readers of this column would follow that route, but sometimes a more immediate impact is required, and that means spending more money for a larger plant.

Container-grown no longer means small, however. O’Donal said there are 25- and 45-gallon plastic pots available, and they can contain some big plants.

Container plants offer some advantages, the most important of which is that you are getting all the roots the plant used to grow to the size it is.

Often, the roots will be pot-bound, meaning that when you remove the plant from the container, the roots will remain in the shape of the container. That is not necessarily bad, as long as the roots are white and healthy, and perhaps have signs of mycorrhizae, living fungi found naturally in the soil that help roots take up nutrition.

When planting the tree, spread the roots out, which may require a trowel or sometimes a knife, so the roots come in contact with more soil.

Because balled-and-burlapped saplings lose a lot of their roots when they are dug up to sell, they require attention and care when you plant them. Shutterstock/Greenseas

With balled-and-burlapped plants, it is surprising how little of the root system is preserved. Those trees have been planted in a field to grow, and the roots are in the top 18 inches of the soil and spread many yards away from the tree’s main stem. The machines used to dig those field-grown trees can’t create a root ball that includes most of the original roots.

“With a 32-inch root ball, you are leaving 70 percent of the roots in the ground,” O’Donal said. “At 20 inches, you leave 80 percent in the ground.”

O’Donal showed participants a chart of how large the root ball should be depending on the caliper (diameter at breast height, about 45 inches) of the tree. A 1.5-inch caliper should have a 24-inch root ball, and a 3-inch caliper should have a 36-inch root ball.

In addition to checking that the root ball is the correct size, buyers should look inside the burlap to check that the roots are white and healthy. It is OK if the tree stem wiggles a little in the root ball – that’s stress from being hauled in a truck – but all the roots should be there.

To make up for the fact that the tree being planted is now trying to grow with much less than half of its original roots, the planting site should be as conducive to growth as possible. O’Donal quoted advice from retired Southern Maine Community College horticulture instructor Rick Churchill: “You don’t dig a hole; you prepare a planting site.”

That site should be 2.5 to 3 times wider than the root ball, and just deep enough to keep the top of the root ball level with the ground. The soil that fills the planting site should be amended with compost and mycorrhizae. At one time, O’Donal didn’t add mycorrhizae, but new research has shown it really helps the tree grow better. You can find bags of the fungi wherever you buy your plants.

Don’t add fertilizer, because you want the tree to settle in and not use its energy to send out new growth, he said. And water. Fill the planting site with water and let it sink in before planting the tree, then water heavily after planting and daily for several weeks after that. From that point on, water it regularly.

You, and whoever is living in your house a half a century from now, will appreciate your care.

Tom Atwell is a freelance writer gardening in Cape Elizabeth. He can be contacted at: tomatwell@me.com.