The question of whether geraniums spread is common, but the answer is complex because the name refers to two distinct groups of plants. Understanding the growth habits of the specific plant you have is the only way to accurately predict its behavior in the landscape.
Clarifying the Geranium Identity Crisis
The plants commonly sold as geraniums are not true members of the Geranium genus. These bedding plants, known for their showy flower clusters, belong to the genus Pelargonium. As South African natives, Pelargoniums are typically treated as tender annuals or houseplants because they cannot survive frost.
The true geraniums are hardy perennials belonging to the genus Geranium, often called Cranesbills. Cranesbills are cold-tolerant and return year after year, forming the backbone of perennial borders. The growth and spreading mechanisms of these two groups are entirely different.
How True Geraniums (Cranesbills) Spread
Plants in the Geranium genus employ two primary methods to increase their footprint: vegetative expansion and seed dispersal. Many vigorous Cranesbills spread horizontally through underground root structures called rhizomes. These specialized stems grow outward beneath the soil, sending up new shoots and creating a dense, self-propagating groundcover.
The Bigroot Geranium (Geranium macrorrhizum) is a well-known example, forming a thick mat of foliage that smothers weeds. Other varieties spread in a clumping manner, slowly expanding the diameter of their central crown. This vegetative spread allows the plant to fill in garden spaces quickly.
The second method is self-seeding, characteristic of many Cranesbill varieties. When the fruit capsule is ripe, the lobes snap open with force, launching the seeds a fair distance from the parent plant. This dispersal mechanism can result in new seedlings appearing several feet away.
To prevent this spread, gardeners often practice deadheading, removing spent blooms before the seeds mature and scatter. Hybrid varieties often produce sterile seeds or self-seed infrequently.
Growth Characteristics of Pelargoniums
The colorful Pelargoniums used in containers and summer bedding displays do not spread like Cranesbills. These plants typically exhibit a mounding, bushy, or upright growth habit. Zonal geraniums (Pelargonium × hortorum) are recognized for their upright, succulent stems and dark leaf markings.
Ivy-leaved Pelargoniums (Pelargonium peltatum) are the exception, displaying a trailing habit ideal for hanging baskets. Even trailing types do not produce rhizomes or runners to expand laterally in the soil. Their size increase is managed by pruning for bushiness and encouraging new stems from the crown.
Since Pelargoniums are grown as annuals, their focus is on producing flowers and seeds, not perennial ground coverage. Propagation is reliably achieved through stem cuttings. They are not considered spreaders in the context of garden takeover.
Controlling Growth and Managing Vigorous Spreaders
Managing the vigorous spread of true Geraniums requires strategic pruning and division.
Pruning and Shearing
For varieties that become sprawling or leggy by mid-summer, the “Chelsea Chop” can be employed in late spring. This involves cutting the stems back by up to half their height to encourage a stockier plant and a denser, delayed second flush of flowers.
A hard shear after the main flowering period can rejuvenate the foliage and prevent the plant from dominating its neighbors. This shearing removes spent flower stems and developing seed capsules, mitigating the risk of unwanted self-seeding. The plant quickly produces a fresh mound of leaves that remain tidy for the rest of the season.
Division
For Cranesbills that spread aggressively via rhizomes, such as G. macrorrhizum, dividing the plant every few years is the most effective control measure. Division should be performed in early spring or early fall by digging up the clump and separating the root ball. This process allows the gardener to discard the older, woody center, promoting healthier growth from the younger sections.