Hydrilla Care Guide (Hydrilla verticillata)
Hydrilla is one of nature's great survivors, and in the aquarium that toughness translates into a fast, forgiving, almost unkillable oxygenator. Sold as long, slender stems ringed with whorls of narrow, finely toothed green leaves, it is a stem plant (an upright plant grown as a leafy stem and propagated from cuttings) and one of the hobby's classic bunch plants for cheap, cheerful cover. Australian hobbyists have a particular reason to like it: unlike most of the vigorous oxygenators, Hydrilla is a genuine Australian native. It is also, as we will see, a plant to keep and dispose of responsibly.
1. Species Overview
The plant sold as Hydrilla is Hydrilla verticillata, the only species in its genus, and a member of the family Hydrocharitaceae – the frog's-bit family, which also gives us Vallisneria and the closely related Egeria and Elodea. It has an enormous natural range across warm and cool fresh waters of Africa, Asia, Europe and Australia; here it is native to the north and east, from the Northern Territory through Queensland into New South Wales. You will also see it called water thyme or Florida elodea, and it carries the synonym Hydrilla lithuanica; our store lists it plainly as "Hydrilla | Hydrilla verticillata", so trade name and botanical name agree. The one thing worth untangling is that Hydrilla looks almost identical to the other whorled-leaf oxygenators – Egeria densa and Elodea canadensis, both also sold loosely as "elodea" – and is easily muddled with them. Two features set it apart: its leaves, borne in whorls of roughly three to eight, have visibly toothed, slightly rough margins and often small spines along the midrib beneath, and it produces turions (small dormant overwintering buds) and underground tubers that its look-alikes do not. It is an obligate submersed plant – living underwater rather than growing up into the air – with trailing stems that reach a metre or two in an aquarium (and far longer in the wild), fanning out into a dense canopy just below the surface.
2. Difficulty Level and Suitability
Hydrilla is easy – genuinely one of the least demanding plants you can buy. Flowgrow rates it "very easy" and Australian retailers "easy" (one scores it three out of ten for difficulty): it asks for no injected CO₂, no special substrate and no strong light, and it shrugs off a very wide span of temperature, hardness and pH. What really sets it apart from the other classic oxygenators is temperature: where Elodea and Cardamine are cool-water specialists that sulk in the warmth, Hydrilla is comfortable in both an unheated coldwater or goldfish tank and a warm tropical community, which makes it one of the most broadly useful bunch plants there is. It is excellent cover for fry and shrimp and a fine first plant. Two honest caveats. It is extraordinarily vigorous, so it wants regular trimming and management rather than neglect. And although it is a beneficial native here, it is a notorious aquatic weed overseas and a declared plant in parts of Australia – which makes responsible ownership and disposal essential, as covered under troubleshooting.
3. Water Parameters
Hydrilla is unusually unfussy, but steady water is always better than water that lurches about. Treat the figures below as working targets rather than hard limits; published sources vary, and this plant tolerates a far wider span than most.
| Parameter | Target range | Notes |
|---|---|---|
| Temperature | 15–27 °C | Exceptionally adaptable. Optimum around 20–27 °C (Flowgrow); tolerant of a very wide span, roughly 4–30 °C, so it suits both unheated coldwater tanks and warm tropical ones. Unlike Elodea, it is not troubled by warmth. |
| pH | 6.0–8.0 | Very adaptable across neutral to alkaline water; tolerant of roughly 5–9 (Flowgrow). Steadiness matters more than the exact figure. |
| GH (general hardness, the amount of calcium and magnesium in the water) | 3–12 dGH | Copes with soft to moderately hard water; hardness is rarely the limiting factor. |
| KH (carbonate hardness, the water's buffering capacity) | 2–12 dKH | Comfortable across a broad band (Flowgrow lists up to 21); tolerates hard, alkaline water and can draw carbon from dissolved carbonates. |
| Nutrients / substrate | Water-column feeder | Sparse roots; takes nutrients mainly through its leaves and stems. A hungry nutrient sponge that helps starve algae. Root tabs optional; likes some nitrate present. |
Hydrilla's sheer adaptability is the headline: equally at home in cold and tropical water and across soft and hard, acidic and alkaline conditions, it suits most Australian tap waters as they come. In practice the limiting factor is almost never water chemistry but space and vigour – this plant will fill whatever room you give it.
4. Lighting Requirements
Hydrilla is undemanding about light, and here the sources genuinely disagree on how much it wants. Using the PAR framework common to our guides – PAR (photosynthetically active radiation, a measure of usable light reaching the plant) of roughly 30–50 counting as low, 50–80 as medium and 80-plus as high for plants – Flowgrow rates its tolerance "low to high", one Australian retailer "moderate to high" and another "low to medium". The practical reading is that Hydrilla grows across the whole range: it copes with genuinely low light, where it tends to stretch into longer, sparser stems, and stays denser and bushier under medium light, its efficient photosynthesis in dim water being one reason it is such a formidable weed abroad. There is no red colour to chase, so there is little reason to push high light; if you do, acclimatise the plant gradually and keep nutrients balanced, or algae will settle on the slower older leaves.
5. CO₂ and Fertilisation
Like all plants, Hydrilla photosynthesises – using light to turn carbon dioxide and water into the sugars it grows on – but it needs no CO₂ cylinder to do so. Flowgrow lists it as happy across a broad carbon range and Australian retailers agree that injection is "not mandatory"; it grows vigorously in a plain, un-injected tank, and added CO₂ is a bonus rather than a requirement. Unusually, in hard, alkaline water Hydrilla can draw carbon from dissolved bicarbonates instead of free CO₂ – a process called biogenic decalcification that sometimes leaves a chalky film on older leaves – which is part of why it grows so freely even without injection. Feed it through the water column with a balanced liquid fertiliser rather than relying on root tabs, since its roots are sparse; as a fast, hungry grower it doubles as a nutrient sponge that helps starve algae. One sensible caution: Hydrilla is not specifically documented as sensitive to glutaraldehyde-based liquid-carbon products (the "Excel" type), but several of its close relatives in the frog's-bit family – Elodea, Egeria and Vallisneria – are known to react to them, so if you use one, dose conservatively; the plant does not need it.
6. Planting and Placement
Use Hydrilla in the background or midground, where its tall stems rise toward the light, or simply leave it floating – both work, and floating bunches make excellent, fast-growing cover for goldfish, livebearer fry and shrimp. There is no rhizome, bulb or crown to fuss over: push small, loose bunches a few centimetres into the substrate and let them root. Because it makes only sparse, thread-like roots it anchors poorly at first, so weight the bunches or use plant lead if your fish like to dig – goldfish in particular will uproot and graze it. Left alone it grows to the surface and forms a dense canopy, so give it room and expect to trim often. It also earns its place in a new, cycling tank, mopping up ammonia and nutrients while slower plants establish.
7. Growth Rate and Propagation
Growth is very fast – among the quickest of any aquarium plant, with healthy stems capable of extending a couple of centimetres a day in good conditions – so plan on regular trimming. Everyday propagation could not be simpler: snip a healthy top a few centimetres long and either replant it or let it float, and it grows on and puts out fresh roots. Hydrilla also has two natural tricks its look-alikes lack – it forms turions (small dormant buds in the leaf axils) and starchy tubers down in the substrate, both of which can sprout into new plants. Those storage organs, together with the plant's readiness to regrow from a mere fragment, are exactly what make Hydrilla so successful – and so hard to eradicate – in the wild, and the reason trimmings and old substrate must be handled thoughtfully rather than tipped outside. Which brings us to the most important part of this guide.
8. Common Issues and Troubleshooting
Most day-to-day problems with Hydrilla are cosmetic and easily fixed. Long, bare lower stems with sparse leaves mean too little light – raise the light, or top the plant and replant the healthy tips. Some lower-stem melt or leaf-drop is normal after planting or a big water change; trim back to firm, green growth and let it recover. Algae filming the older leaves usually means light is outrunning nutrients; trim the worst and rebalance. In truth the commonest "problem" is simply the plant's vigour: left untrimmed it will fill the tank and shade slower neighbours, so treat frequent pruning as routine rather than a fault.
The one issue that is not cosmetic is Hydrilla's status as an aquatic weed, and it must not be glossed over. In Australia the picture is nuanced. Hydrilla is native to the mainland – occurring naturally from the Northern Territory through Queensland and New South Wales – and Weeds Australia notes that it is rarely a problem in healthy Australian waters and is usually ecologically beneficial, becoming weedy mainly where nutrient pollution tips the balance; it is not a Weed of National Significance. Overseas the story is very different: Hydrilla is regarded as one of the world's worst aquatic weeds, a serious and heavily regulated pest in the United States and an invader of waterways in New Zealand, Fiji and the Americas. Within Australia it is a declared weed in Western Australia and Tasmania – Tasmania, where it has not naturalised, prohibits its importation, sale and distribution under the Biosecurity Act 2019 – so keepers in those states in particular need to check the rules before buying or keeping it. Because Hydrilla regrows from the smallest fragment, turion or tuber, you must never release it – or its trimmings, or tank water or old substrate containing it – into rivers, creeks, dams, drains or stormwater. Dispose of any excess by bagging it for the general rubbish, or by drying it out completely or deep-composting it well away from any waterway. None of this is legal advice – rules differ by state, territory and even local council, and they change – so please check the current requirements for your own area (your state department of primary industries or agriculture, or weeds.org.au, is a good place to start) before buying, keeping or moving this plant.
A Note on the Ranges Above
The parameters and other figures in this guide are a general guideline only. Every planted aquarium is different, and the ideal balance for Hydrilla (Hydrilla verticillata) depends on your lighting, your substrate, your other plants and whether you run CO₂. For help working out tank volume and the right dose of fertiliser, plant food and treatments for your system, have a look at our Aquarium Calculators.
Disclaimer
This guide is provided for general educational purposes only. It is not a substitute for professional advice, and we cannot guarantee outcomes. Individual plants vary, and the right conditions for your system depend on your lighting, your equipment and the other livestock and plants you keep. If you are unsure about anything, please get in touch with our team before proceeding.