Propagating Limnobium spongia (American Frogbit) by Runners and Division

How to multiply floating American frogbit through daughter plants on runners and simple division, plus thinning to keep this fast spreader from shading the tank.

Overview

Limnobium spongia, American frogbit, is a floating aquatic plant with heart-shaped or nearly rounded leaves 2-7 cm across. Floating leaves carry a thick layer of spongy aerenchyma on their underside, while emersed leaves lack this spongy tissue. With long trailing roots it provides excellent surface shade and cover, making it popular for blackwater and biotope tanks.

In its native southeastern United States it grows in sloughs, ditches and swampy forests in slow-moving or stagnant water, so it thrives in calm, still surfaces.

Propagation Method: Offsets on Runners

American frogbit propagates by sending out runners, spreading like a connected web of plants. Daughter plants form naturally along these runners without any manual intervention, so the colony multiplies on its own. To deliberately propagate, you simply separate established daughter rosettes, a form of division.

Step-by-Step

Let the parent plant grow until it sends out runners with small daughter rosettes attached.
Wait until each daughter has its own leaves and a few roots of its own.
Gently pinch or snap the runner to separate the daughter from the parent.
Float the separated plant back on the surface in a calmer area so it can settle.
Remove whole connected clumps in one piece when thinning, as the web lifts out easily.

Conditions for Healthy Growth

Placement: free-floating on a calm, still surface.
Temperature: 18-28 C.
pH 5.5-7.5, GH 2-14.
Lighting: medium light; floating plants grow very quickly and absorb nitrogen from the water.
Avoid strong surface agitation, which wets and rots the floating leaves.

Maintenance

Thin the colony regularly, roughly every couple of weeks, by scooping out connected clumps. Do not let it cover the entire surface, or it will out-shade the plants below and reduce dissolved oxygen in the water.

Common Challenges

Inside the tank the main issue is the opposite of weakness: overgrowth. Frequent thinning keeps light reaching plants below and prevents oxygen depletion under a closed surface mat.