Geraniums, particularly the popular bedding varieties, exhibit a wide range of mature sizes determined by genetics and influenced by the growing environment. The plant commonly called a geranium is actually a Pelargonium, a tender perennial grown as an annual in most climates. True Geraniums (Cranesbills) are typically smaller, hardy perennials. Understanding the size potential of these Pelargonium types is essential for effective garden planning.
Size Variation by Geranium Type
The ultimate dimensions of a geranium are dictated by its species or hybrid group. The Zonal Geranium (Pelargonium x hortorum) is the most common variety, typically forming a bushy, upright mound. These plants generally reach a mature height and spread of between 1 and 3 feet, though many compact cultivars remain closer to 12 to 18 inches tall.
Regal Geraniums, or Martha Washington Geraniums (P. domesticum), share a similar upright habit, usually maturing to a height of 1 to 3 feet with a spread of 1 to 2 feet. These varieties have dense foliage and large, showy flowers. Scented Leaf Geraniums, such as P. graveolens, are often the largest, naturally growing into multi-branched, erect subshrubs. Certain varieties can reach up to 4 to 5 feet in height and 2.5 to 3 feet in width under ideal conditions.
Ivy Leaf Geraniums (P. peltatum) display a different growth pattern, focusing on horizontal spread. While their stems may only be 10 to 12 inches tall, they are trailing plants whose stems can cascade up to 3 feet or more. This sprawling nature makes them suited for hanging baskets and window boxes, where length is the primary measure of size.
Environmental Factors Affecting Growth
Plant size is not fixed; environmental conditions determine whether a geranium reaches its full genetic potential. Light exposure significantly influences the plant’s shape and density. Geraniums grown in full sun exhibit dense, compact growth with shorter internodes (the spaces between leaf sets).
Plants receiving insufficient light will stretch, resulting in a leggy, taller appearance as stems elongate in search of brighter conditions. Temperature also influences growth rate. High average daily temperatures, especially exceeding 90°F, slow growth and limit flower development in Zonal varieties. Cooler night temperatures, ideally between 50°F and 60°F, promote robust flowering and maintain a stocky habit.
The nutrient balance in the soil directly impacts the ratio of foliage to stem size. Over-fertilization, particularly with high-nitrogen formulas, encourages excessive vegetative growth, leading to larger leaves and longer stems at the expense of flower production. Conversely, a lack of essential nutrients results in a smaller, stunted plant. Well-drained soil is necessary, as waterlogged conditions impede root development and can cause root rot, limiting the plant’s size and health.
Techniques for Size Management
Gardeners have several methods to control the size and shape of a geranium, especially in container gardening. The simplest technique is pinching, which involves removing the soft growing tip from a stem just above a leaf node. Pinching redirects the plant’s energy, encouraging new side shoots to develop, resulting in a bushier, denser, and shorter plant.
The container size is a tool for size restriction, as smaller pots physically limit root growth. When root expansion is restricted, the plant’s above-ground growth is suppressed, leading to a more compact specimen. For plants that have become woody or overly large, hard pruning serves as a rejuvenation technique. This involves cutting back the stems by up to a third or more, often done when preparing plants for overwintering or before a new growing season.