Hygrophila 101

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Hygrophila History, Biology, and Taxonomy - AKA how Petunias, Australia, and Diabetes are Related

Taxonomy

Hygrophila (sometimes called swampweed) is the more popular of the two aquatic genuses in the Acanthaceae family, which makes it a "cousin" to other commonly kept plants like Acanthus (Bear's Breeches), wild petunias (Ruellia), and the polka dot plant (Hypoestes phyllostachya).

There are about 100 Hygrophila species currently recognized. Botanists and taxonomists are in the process of re-evaluating how to define the genus and its species, though, so the number might change in the near future.

History (Or, how I came to want to have a friend name a plant after me)

A Scottish botanist named Robert Brown (1773-1858) first formally described and organized the plants, along with countless others, thanks to his talent with a microscope and several exploratory expeditions. Fun fact: Brown was able to do this partially because of his collaboration with cartographer and explorer Captain Matthew Flinders, who was the first to call Australia "Australia" and had a famous ship's cat named Trim. The two got along so well, Brown actually named 14 species of citrus trees after him (Flindersia).

Just like with other plants in its family, scientists are interested in Hygrophila for potential medical uses. Hygrophila auriculata (AKA kulekhara; neeramulli; Hygrophila longifolia; Hygrophila spinosa) has been used in Ayurvedic medicine for years and may be useful in treating and managing diabetes. There's also evidence it can help increase pain tolerance in neuropathy patients. 

Biology

This is one of those plants that can grow just about anywhere, so you can find it naturally in most parts of the world. Because of its undemanding needs, it is also invasive in several places.

So be sure to check your agriculture laws before you purchase or transport it in your area. And if you decide you hate this awesome plant for some reason, make sure you dispose of it properly. Hygrophila polysperma in particular has been a nuisance in southern waterways. 

Hygrophila is also a plant of interest due to some species exhibiting something called heterophylly, which means the leaves grow differently depending on their environment or life stage. Water Wisteria (Hygrophila difformis) is a great example of this. Grown submerged, you get the ruffled, irregular leaf shape that gives the plant the "difformis" part of its name. Grown above water, though, the leaf morphology changes to a more elliptical look. 

Two forms of Hygrophila difformis. The one on the left was grown submerged, while the one on the right was grown above water

Best Hygrophila Species for your Planted Tank

Hygrophila cordata

Hygrophila corymbosa - several cool varieties (AKA Nomaphila stricta)

• Cherry Leaf

• Compacta
• Parawitota

Water Wisteria (Hygrophila difformis) AKA Ruellia difformis, Synnema triflorum

Hygrophila "Nachimale"

Hygrophila pinnatifida

Hygrophila salicifolia

Hygrophila siamensis

Hygrophila stricta

How to Not Murder Hygrophila

These plants are fairly hardy (varying between the species), and even a basic care routine can keep them looking sharp without a lot of effort. So if you're the type of person who blinks wrong at a plant and if shrivels up and dies in protest, Hygrophila might be a good choice for you.
Temperature range: highly adaptable. Can tolerate a wide range of conditions (~68°F to low 80's). 
Fertilizer needs: low (most species thrive with it, but are more forgiving of low nutrient availability than other aquarium plant species). Ditto on CO2.

Light needs: Most Hygrophila species will survive in lower quality light (especially if you float them). As always, better light for appropriate amounts of time will help them grow more quickly, but these aren't the kind of plants where if you don't drop a house payment on a light setup you don't have a chance.

Most plants need between 6-12 hours of light each day, depending on your setup, species, plant age/growth stage, etc. We always recommend a cheap light timer to make this easier!

pH: Naturally grows in pH of ~6.5-7.8, but is a very adaptable plant 
Tank mates: any fish or invertebrates that can live in the tropical/subtropical temp range listed above. Cichlids, turtles, and other plant destroyers may pose a challenge, though.

Dissolved Oxygen (DO): There's not a single plant we won't recommend running with surface agitation and/or an airstone. 
Placement: varies by species. There are good choices for every area of your tank, including floating. Several of these will grow just fine with no substrate.
Substrate: pick your poison. Hygrophila is suited to most substrates, or no substrate 
Buying tip: Consider buying them grown underwater, because the transition from above water to underwater can sometimes be rough on them. They also tend to drop leaves during shipping, but are otherwise easy and grow these back quickly. 

Happy Hygrophila growing, and tank on!