Plant choice decides more about a jar than any other single decision. Plants make the oxygen, take up the waste, and give shrimp somewhere to graze. They also compete with algae for the same light and nutrients. A jar with the right plants is halfway to stable before the first animal goes in.
The plants below are the ones that work in a small, low-tech jar: no CO₂ injection, no fertiliser dosing, no powerful light.
What a jar plant needs
Three things matter more than species:
Low light tolerance. Most jars sit on a desk or a windowsill, not under a grow light. Plants that need high light will fade, and the light you add to compensate will feed algae.
Slow or controlled growth. A plant that doubles every week will fill the jar and starve everything else of light. You can trim it, but a first jar is easier with plants that grow slowly.
Ability to feed from the water. In a jar with gravel and no soil, plants that take nutrients through their leaves do better than heavy root feeders. Java moss, hornwort, and water sprite are all water-column feeders.
The reliable five
Java moss (Taxiphyllum barbieri)
The best first plant. It grows on stone and wood, tolerates low light, and gives shrimp a place to graze. It does not need soil. Tie it to a small stone with cotton thread, or let it sit loose on the substrate. In a jar it will slowly spread into a soft green cushion.
Java moss can also become a trap for detritus. If it looks brown inside, siphon gently with a turkey baster during a water change.
Anubias nana (Anubias barteri var. nana)
Broad, dark green leaves and a very slow growth rate. Anubias is one of the few plants that looks better in a jar than in a big aquarium because the scale works. Keep the rhizome above the substrate. It will grow attached to a stone or a piece of wood.
Anubias is expensive relative to moss, but one small plant is enough.
Hornwort (Ceratophyllum demersum)
A fast grower that floats or anchors loosely. Hornwort is excellent at pulling nutrients from the water and releasing oxygen. It can also shed needles when it is stressed, which makes a mess. If it starts dropping needles, check the temperature and the light.
Trim it regularly. Hornwort can double in a few weeks in a bright jar.
Marimo (Aegagropila linnaei)
A slow-growing green sphere, actually a colony of algae, not a moss. It is very tolerant of low light and low nutrients. It does not filter water quickly, but it is a stable, long-lived resident that shrimp like to sit on. See the marimo care guide for the details.
Water sprite (Ceratopteris thalictroides)
Feathery, fast, and useful in a new jar because it takes up nutrients quickly. It can be planted or left floating. It grows large; trim it before it covers the surface.
Water sprite is a good “first responder” plant: if nutrients are climbing, it will use them. The trade-off is that it can take over a small jar.
Floating plants
Duckweed (Lemna minor) and salvinia are common floating plants. They are excellent at nutrient export and provide shade, but they multiply quickly and block light from the plants below. Duckweed is almost impossible to remove completely once it is established.
If you use floating plants, keep a small area of the surface clear and remove excess every week. In a small jar, I would choose one floating plant or none.
Plants to avoid in a small jar
Hairgrass and other high-light carpet plants. They need strong light and often fail in a jar. The light required to keep them alive will grow algae.
Large Amazon swords and other big rosette plants. They outgrow the jar and their roots need more substrate than a jar provides.
Fast-growing stem plants in a low-light jar. They will either stall or grow leggy and pale. If you want a stem plant, choose hornwort or water sprite instead.
Plants collected from a pond. They bring snails, leeches, and algae. If you must use local plants, quarantine them in a separate container for a few weeks.
How many plants?
A useful rule: the jar should look slightly overplanted when you finish. Plant mass is the engine of the system. For a 2–4 litre jar, a golf ball of Java moss, one small anubias, and two or three stems of hornwort is a reasonable start. That is more plant than it sounds.
Do not fill every square centimetre on day one. Leave room for growth and for the shrimp to move.
Planting without soil
Most jar plants do not need soil. Gravel or sand is enough for anchorage, and the water column supplies the nutrients. If you want to use a nutrient-rich substrate, put a thin layer under the gravel and cap it well; a jar has no filter to clear the cloud that a disturbed soil layer creates.
For anubias and moss, attach the plant to hardscape rather than burying it. For hornwort and water sprite, floating is fine.
Trimming
Trimming is not just cosmetic. Removing growth removes nutrients from the jar. Cut stems above a node and remove the cuttings; do not leave them to rot. Thin Java moss if it looks dense and brown inside. Remove yellowing leaves before they become detritus.
The Pondlet shop has cosmetic plants for the virtual jar, but in a real jar the rule is simpler: grow what you can see through, and keep it trimmed.