Dwarf Hairgrass, primarily Eleocharis parvula and Eleocharis acicularis, is a popular fine-leaved aquatic plant used to create a lush, underwater lawn. Aquarists aim for a short, dense carpet, but the grass often stretches upward, defeating the purpose of a foreground plant. The final height depends on the specific species planted and the environmental conditions maintained within the aquarium.
Understanding the Height Range of Dwarf Hairgrass Species
The term “Dwarf Hairgrass” is used broadly, causing confusion about expected growth height due to distinct species having different growth habits. The species most desired for a short carpet is Eleocharis parvula, which typically maintains a compact height of 1 to 3 inches (2.5 to 7 centimeters) under optimal conditions.
Eleocharis acicularis is a taller species, commonly sold, that can reach 4 to 6 inches (10 to 15 centimeters) when fully submerged. The smallest variety, Eleocharis acicularis ‘Mini’, naturally stays closer to 1 inch (2.5 cm). When any hairgrass stretches significantly beyond its typical maximum height, it is reacting to insufficient resources by growing vertically to reach a perceived necessity.
Environmental Factors That Control Vertical Growth
The primary mechanism influencing vertical growth is the plant’s response to light and carbon availability, which determines whether it focuses on horizontal spread or vertical reach. Low light intensity causes etiolation, where the plant rapidly elongates its blades to reach the light source, resulting in tall, thin growth. High Photosynthetic Active Radiation (PAR) delivered to the substrate encourages the plant to stay compressed and dense, eliminating the need to stretch for light.
Carbon dioxide (CO2) availability also significantly determines the plant’s growth pattern. Dwarf Hairgrass, like many aquatic plants, relies heavily on dissolved CO2 for efficient photosynthesis. When CO2 is deficient, the plant conserves energy by reducing its overall growth rate and may prioritize vertical growth to gain a competitive edge for carbon in the water column.
Supplemental CO2 coupled with high light maximizes the plant’s photosynthetic output. This energy surplus is channeled into producing stolons, or horizontal runners, which spread along the substrate to colonize new areas, rather than increasing vertical height. A balanced nutrient profile is also necessary; excess macronutrients, such as Nitrates, can encourage rapid, tall growth. Controlling the balance of carbon, light, and nutrients is the most effective way to signal the plant to prioritize lateral expansion.
Maintaining a True Carpet: Trimming and Density
Achieving and maintaining a low-growing carpet requires specific physical interventions, even when environmental factors are optimized. Initially, planting density is crucial. The plant should be separated into very small tufts, or nodes, and planted closely together, spaced about 1 to 2 inches apart. This immediate proximity signals to the plant that the space is already somewhat occupied, prompting it to spread outward via runners instead of upward.
Once the carpet has begun to fill in and reaches a height of about 2 to 3 inches, regular trimming becomes necessary to maintain the desired lawn effect. This is accomplished using the “lawnmower” technique, where the entire carpet is cut back aggressively with long, straight or wave scissors to a uniform height of approximately 0.5 to 1 inch above the substrate. Trimming stimulates the rhizomes to produce new, shorter blades and more horizontal runners, effectively rejuvenating the carpet and making it denser.
The substrate itself is a physical factor in maintenance. Dwarf Hairgrass is a deep-rooting plant that develops an extensive network of rhizomes. A substrate depth of at least 2 to 3 inches is optimal, as it provides a secure anchor for the root system, preventing the buoyant grass from being easily uprooted by water changes or fish activity. Trimming should be performed before the carpet becomes too dense and tall, which can cause the lower layers to shade and rot, leading to the carpet lifting away from the substrate.