The ivy covering the outfield walls at Wrigley Field is Boston ivy (Parthenocissus tricuspidata), a vigorous climbing vine native to East Asia. It shares the walls with a secondary planting of bittersweet (Celastrus), though Boston ivy has always been the dominant and most visible species. Despite the name, Boston ivy is not a true ivy at all, and understanding what it actually is helps explain why it thrives on a ballpark wall in Chicago.
Boston Ivy Is Not a True Ivy
The name trips people up. When most people hear “ivy,” they picture English ivy (Hedera helix), the dark-green, lobed-leaf evergreen that creeps along the ground and up buildings across much of North America and Europe. Boston ivy is a completely different plant. It belongs to the grape family (Vitaceae), making it a closer relative of the grapevine on a vineyard trellis than of the English ivy on a college building. Its leaves are broad, three-lobed, and deciduous, meaning they drop every autumn. That seasonal leaf loss is part of what makes Wrigley’s walls so visually distinctive: lush green in summer, blazing red and orange in October, and bare brown vines through winter and early spring.
The “Boston” in its name comes from its popularity on brick buildings in the northeastern United States during the late 1800s, when the plant was imported from Japan and China. The species spread rapidly as an ornamental, especially on university campuses in the Ivy League, where it still grows on many of the buildings that gave those schools their collective nickname. So while Wrigley Field’s vines are often called “the ivy” in baseball shorthand, they are technically a grape-family vine, not a member of the Hedera genus at all.
How Boston Ivy Clings to Walls
One reason Boston ivy works so well on a vertical outfield wall is its climbing mechanism. Unlike English ivy, which uses aerial rootlets to grip surfaces, Boston ivy produces specialized tendrils that end in small adhesive discs. When a tendril tip makes contact with a wall, the disc cells undergo rapid division, forming finger-like projections that press against the substrate. These projections are then cemented in place by an adhesive secreted from the disc’s peripheral cells, filling the space between the disc and the wall surface and even penetrating into the tiny crevices of brick or concrete.
Research into these adhesive discs has shown just how effective the system is. The adhesive fills not only the contact zone between the disc and the wall but also the spaces between the outer cells of the disc itself, creating a remarkably strong bond.
1Applied Microscopy. Structural Changes of Adhesive Discs during Attachment of Boston Ivy Electron microscopy studies of Boston ivy tendrils confirm that the cell walls in the adhesive region develop pockets that fill with the secreted glue, which eventually merges into a continuous layer locking the disc to whatever it has touched.2Canadian Journal of Botany. Adhesion of the Boston ivy tendril
This self-adhesive ability means the plant needs no trellis, no wire framework, and no human assistance to scale a wall. The vine simply grows upward and outward, anchoring itself as it goes. At Wrigley Field, the brick outfield wall provides an ideal surface for those adhesive discs to grip. The result is a living wall covering that regenerates itself year after year with relatively little structural support from the grounds crew.
How the Ivy Got to Wrigley Field
The ivy was planted in 1937 by Bill Veeck, then a young executive working in the Cubs’ front office under team owner Philip K. Wrigley. Veeck would later become one of baseball’s most colorful franchise owners, but at the time he was an ambitious 23-year-old looking for ways to make the ballpark feel more inviting. He had the idea to cover the outfield walls in greenery, partly to soften the look of the aging brick and partly to create an atmosphere that set Wrigley apart from every other stadium in professional sports.
Veeck originally planted both Boston ivy and bittersweet. The bittersweet was intended as a fast-growing companion that would fill in gaps while the slower-starting Boston ivy took hold. Over the decades, the Boston ivy proved to be the hardier and more dominant of the two, and it now accounts for the vast majority of what fans see on the walls. Some bittersweet vines remain woven through the planting, but they are a minor presence.
The planting was not universally popular at first. Some baseball purists thought the vines would interfere with play, and there were concerns about maintenance. But as the ivy matured and became synonymous with the ballpark, it turned into one of the most recognized features in American sports. Today, the ivy-covered walls are so central to Wrigley Field’s identity that any renovation of the stadium has had to work around preserving them.
The Seasonal Cycle on the Outfield Walls
Because Boston ivy is deciduous, the outfield walls at Wrigley go through a dramatic transformation across the baseball season. On Opening Day in late March or early April, the walls are mostly bare: a tangle of woody brown stems clinging to the brick, with maybe the first hints of new leaf buds starting to unfurl. By mid-May, the leaves have filled in enough to create a recognizable green backdrop. At peak summer growth in July and August, the wall is a dense, uniform curtain of broad green leaves, sometimes six inches deep or more.
Then autumn brings the show that photographers love. Boston ivy is one of the most reliably colorful deciduous vines in temperate climates, and the Wrigley walls turn vivid shades of scarlet, crimson, and orange as chlorophyll breaks down and other pigments emerge. If the Cubs are playing into October, the postseason games take place against a wall that looks like it is on fire. By November, the leaves have dropped entirely, and the walls return to their bare winter state.
This cycle is worth noting because it directly affects gameplay. Early-season and late-season games feature walls with much less cushion from foliage. A ball that disappears into thick July ivy might simply bounce off exposed brick in April. The grounds crew monitors the growth carefully each spring, and the transition from bare wall to leafy wall is one of the unofficial markers of the baseball calendar for Cubs fans.
Ground Rules and the Ivy
The ivy at Wrigley Field is not just decorative; it has its own set of ground rules that affect the outcome of plays. If a batted ball lodges in the ivy and the outfielder cannot retrieve it immediately, the batter is awarded a ground-rule double. The fielder signals this by raising both hands above his head to alert the umpires that the ball is stuck. If the fielder reaches into the ivy and pulls the ball out, play continues as normal.
This creates a genuinely unique tactical situation. An outfielder who sees a ball roll toward the ivy-covered wall has to make a split-second decision: go after the ball aggressively and risk having it bounce away while he’s digging through vines, or hold up, signal for the ground-rule double, and concede the extra base. The density of the foliage matters here. In the thick of summer, a hard-hit ball can vanish into the leaves as if swallowed. In April, when coverage is sparse, almost nothing gets truly lost.
There have been memorable moments built around this rule. Outfielders have occasionally pretended the ball was lost in the ivy when they had already pocketed it, hoping to deceive baserunners. The tactic is illegal and will draw an ejection if caught, but the temptation exists precisely because the ivy is dense enough to be genuinely opaque during peak season.
What Climbing Vines Actually Do to Buildings
A common worry about any vine-covered structure is whether the plant damages the wall underneath. The answer depends heavily on the species, the wall material, and the climate. Boston ivy’s adhesive discs bond tightly to masonry, and removing old vines can pull off paint or small chips of morite. But research on climbing plants, including Boston ivy, suggests the relationship is more protective than destructive in many cases.
One well-documented benefit is thermal buffering. Field measurements of several climbing species, including Boston ivy and English ivy, show that a leaf canopy about 200 millimeters deep produces roughly the same shade temperature regardless of species. Modeling based on those measurements found that leaf cover on equator-facing walls can reduce indoor temperatures by a steady five degrees Celsius in hot climates during summer.
3Energy and Buildings. Thermal improvement by means of leaf cover on external walls — A simulation modelIn colder climates, climbing vines can also protect masonry from frost damage. English ivy growing on limestone walls has been shown to act as a thermal blanket that reduces the frequency and severity of freeze-thaw cycles reaching the stone surface. Because ice-driven breakdown is one of the main ways masonry deteriorates in cold, wet climates, the vegetation layer can actually slow physical weathering. The foliage also acts as a rain shield, limiting how much moisture reaches the stone in the first place, which further reduces the risk of frost damage.
4Scientific Reports. Thermal blanketing by ivy (Hedera helix L.) can protect building stone from damaging frostsAt Wrigley Field, Chicago winters are brutal, with frequent freeze-thaw cycling from November through March. During those months, the deciduous Boston ivy has shed its leaves and offers less insulation than an evergreen vine would. But during the shoulder seasons of autumn and spring, the remaining foliage and the dense mat of stems still provide some buffering. And during summer, the leaf canopy shields the brick from direct sun and driving rain, both of which contribute to long-term masonry wear.
Maintaining the Wrigley Ivy
The ivy at Wrigley Field is maintained by the ballpark’s grounds crew with a level of care that goes well beyond typical landscaping. During the growing season, the vines are watered through a drip irrigation system embedded behind the walls. The crew trims the ivy to keep it at a consistent depth and to prevent it from growing over the top of the wall or encroaching on the warning track. Sections that die back or thin out are replanted with new vine starts, sometimes grown from cuttings taken from the existing planting to preserve genetic continuity.
The 2014-to-2017 renovation of Wrigley Field posed a particular challenge. The project required extensive work on the outfield wall structure, which meant the ivy had to be carefully removed, preserved off-site, and replanted after construction was complete. Sections of the original vine stock were kept alive in greenhouses and nursery beds during the years of construction, then reintroduced to the walls. The fact that the team went to this effort rather than simply replanting with new nursery stock speaks to how much symbolic weight the original vines carry.
Winter dormancy is actually an asset for maintenance. With the leaves gone and the plant in a resting state, the crew can inspect the wall surface, repair mortar joints, and address any areas where the vine’s adhesive discs have caused superficial damage. This annual window is when most structural maintenance on the outfield wall happens, because doing it during the growing season would mean tearing away healthy vines.
Why Other Stadiums Have Not Copied the Look
Given how iconic the Wrigley ivy has become, you might wonder why every ballpark in the country has not tried something similar. A few have made gestures in that direction. Some minor-league stadiums have planted ivy or other climbing vines on portions of their outfield walls. But no other major-league park has adopted a full ivy covering, and there are practical reasons for that.
Modern outfield walls are padded with thick foam covered in vinyl, designed to cushion the impact when an outfielder runs full-speed into the wall. Wrigley’s brick wall predates this safety standard, and the ivy itself serves as a partial cushion, though not one that would satisfy current ballpark design guidelines if the stadium were built from scratch today. A new stadium trying to replicate Wrigley’s look would have to find a way to grow vines on top of modern padding materials, which do not offer the porous, rigid surface that Boston ivy’s adhesive discs need to grip. The vine would essentially have nothing to cling to.
There is also the maintenance commitment. A living wall covering requires irrigation infrastructure, seasonal trimming, winter inspection, and occasional replanting. It introduces a biological variable into a sport that prefers uniform playing surfaces. And it creates the ground-rule ambiguity described earlier, which most teams and leagues would rather avoid. Wrigley gets away with it because the ivy was there long before modern stadium standards existed, and removing it would be culturally unthinkable.
Bittersweet and the Supporting Cast
While Boston ivy dominates the conversation, the bittersweet vines planted alongside it in 1937 are still part of the wall’s ecology. Bittersweet is a twining vine, meaning it wraps around supports rather than adhering directly to flat surfaces. At Wrigley, the bittersweet intertwines with the Boston ivy stems and uses them as scaffolding. Its leaves are narrower and less showy than Boston ivy’s broad, three-lobed foliage, so it blends into the background during the growing season.
Where bittersweet makes itself known is in autumn and winter. The plant produces clusters of bright orange-red berries that split open to reveal a yellow interior. After the Boston ivy leaves have dropped, these berries can sometimes be spotted among the bare brown stems, adding a subtle splash of color to the otherwise dormant wall. For most fans watching a game on television, the bittersweet is invisible. But anyone who walks up close to the outfield wall during a ballpark tour in late fall will notice the occasional berry cluster tucked into the vine matrix.
Bittersweet is considered invasive in many parts of the eastern United States, particularly the Asian species Celastrus orbiculatus. In a naturalized setting, it can strangle trees and crowd out native plants. At Wrigley, confined to a narrow strip of brick wall and managed by a dedicated grounds crew, it poses no ecological threat. But the presence of an aggressive vine species in such a controlled setting is a reminder that the outfield wall is essentially a curated garden, not a wild ecosystem.