Is your yard a vast blank lawn that runs from fence to fence? Do you have awkward spaces to the side of your house or garage that don’t get much use? Do you rarely sit outside because you feel too exposed or uncomfortable or there’s nothing much to see?

The solution may be to create what designers call “garden rooms.” They are a way to tame an outdoor space by thoughtfully dividing it into smaller areas, each of which has its own purpose, design, plantings and mood.

“An outdoor space can become more usable and more inviting if you divide it,” said Susan Jacobson, head of site planning and design at The Morton Arboretum in Lisle and a landscape architect.

It’s an approach used in the Arboretum’s lush and colorful new Grand Garden, opening to the public at 1 p.m. Sunday. The long, 2-acre rectangle is divided into three spaces: a central plaza, a serene space suited for celebrations, and the Joy of Plants Garden, which shows off the range of plants suitable for Midwestern gardens. The Joy of Plants Garden is itself divided into six garden rooms, each with its own seating area and a different array of plants.

Garden rooms can have varying functions as well as different looks. For example, a patio or eating space may be defined by plants or structures that provide a sense of enclosure. A play area for children might be screened from a secluded space for adult relaxation. “Garden rooms can allow people in the family to use the space in different ways,” Jacobson said.

How do you decide where to draw the line? The growing conditions of the garden are your first guide. For example, the shady part might be the spot for a tranquil seating space. Another area out in the sun might become a vegetable garden or a cutting garden for flowers that need a full day’s light.

Boundaries may suggest themselves. “It’s easy to build off a corner, such as the corner of the house or the corner of a garage,” Jacobson said. A tree or a large shrub can establish a separation. A change in the surface underfoot, such as grass or pavers, can also help define a garden room. Furnishings, such as a dining table or a seating arrangement, can divide space just as they do inside an open-plan home. The area around a patio or deck easily becomes a zone.

Rooms in the garden don’t need solid walls. The divisions may be formed by tall plants such as shrubs or ornamental grasses, large containers, or simply a shift in the design style or color palette from one area to the next. “You can decide on the degree of separation you want,” she said.

In a very large property, strong divisions, such as tall hedges, can make a relaxing background to divisions of space. In a small yard, such barriers could create a cutup feeling. Not all yards need to be divided; when the space is tight, a single well-planned garden room might be enough.

When long-lived plants such as trees and shrubs shape lasting boundaries, Jacobson said, you can play with the perennials, annuals and containers that surround them. “The mood of a garden room can shift with just a change in colors,” she said, and redoing one garden room is not as daunting as replanting the whole yard.

Even if you don’t tinker, garden rooms will change over time as plants grow and change. For example, one of Jacobson’s favorite spaces in the new Grand Garden is the Juniper Terrace, a space at the far end of the Celebration Garden. As juniper shrubs grow over the next few years, it will become an increasingly secluded space within the garden’s grand vista.

“Garden rooms give you a structure that remains while plantings develop from season to season and year to year,” she said.

For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plant clinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.