Passive hydroponics — or passive hydro, as most growers call it — is the simplest way to grow indoor plants in water rather than soil. No pumps, no aerators, no greenhouse setup. Just a plant, a glass vessel, water, and the right fertiliser.The roots draw water and nutrients directly from the vessel — no soil, no pumps, no daily fuss.

The result is a low-maintenance, mess-free, genuinely beautiful way to grow indoor plants — and you get that distinctive look of roots suspended in a glass vessel that most plant photos online don't actually do justice.

A quick clarification before going further: this guide is about passive hydro specifically — your plant's roots suspended directly in a vessel of water. That's different from LECA or semi-hydroponics (where roots grow in clay pellets with water at the base). Both are great, but they're different setups with different care patterns. This guide stays in the simpler glass-vessel territory.

Worth saying upfront: passive hydro isn't a productivity hack or a way to grow plants faster than soil. You'll see commercial hydroponics setups claiming dramatic yield increases, but those are industrial pump-driven systems growing lettuce and tomatoes. For home indoor plants in glass jars, passive hydro is a beautiful display method that's slightly lower-maintenance than soil. That's the honest version.

A note on the Method

The Plant Runner Method (Light → Water → Mix → Feed) was built around soil-grown indoor plants. Passive hydro is the one context where the framework needs adjusting — there's no mix. The Light → Water → Feed pillars all still apply, but Mix gets replaced with Vessel and water management, which becomes the load-bearing decision instead.

That's the structure below.

Plants that work in passive hydro

Some indoor plants take to water beautifully. Others tolerate it for a while and then slowly decline. The ones that thrive long-term:

• Pothos / Devil's Ivy — the easiest. Pothos in water can live for years with the right care.
• Philodendron — most species work well, particularly heart-leaf
• Peace Lily — actually flowers in water in good light
• Spider Plant — fast-rooting, easy to propagate from water
• Chinese Evergreen (Aglaonema) — tolerates water well, prefers warmer rooms
• Monstera — adapts to water but grows more slowly than in mix; treat it as a long-term display piece rather than a plant that'll get bigger

What's the pattern? Most are aroids — plants from the Araceae family, evolved for environments where root contact with water is normal. They have the biological flexibility to grow either in soil or suspended in water. The full aroid family overview is worth a read if you want the deeper context.

Plants that don't take to passive hydro: succulents and cacti (root architecture isn't built for it), most flowering houseplants (don't adapt well to standing water long-term), most ferns (prefer moisture-retaining mix over a static reservoir), and fruiting plants like tomatoes (need active pump systems, not passive).

One thing worth knowing: plants grown in water develop a different type of root system over time — finer, more adapted to water uptake than the thicker structural roots that develop in soil. This is why moving an established hydro plant back into soil can temporarily shock it, and why a soil-grown plant takes a few weeks to settle when first transitioned to water. Neither is a problem; they're just different growing modes, and the plant needs time to switch between them.

Light

The same Shadow Test that applies to soil-grown plants applies in passive hydro: hold your hand about 10cm above a leaf at midday. A crisp, defined shadow means good light — your plant will push consistent growth. A soft, fuzzy shadow means lower light, and growth will slow.

Bright, indirect light is the sweet spot. Avoid harsh direct sun on a passive hydro vessel — the sun heats the water (which stresses the roots) and dramatically accelerates algae growth in the vessel. A hot afternoon-sun window is the worst placement.

If your space is genuinely dim, a simple LED grow light positioned above the plant works well. Passive hydro plants respond to supplemental lighting just like soil-grown plants do.

Water and the vessel

The vessel choice matters more than people realise. A few practical principles:

Water level: fill to about one-third to halfway up the vessel — never to the top. The roots still need access to oxygen above the waterline. Submerging the entire root system in stagnant water suffocates the plant from the bottom up — there's no air access for the root tissue to breathe.

Vessel material: opaque vessels (ceramic, dark glass, terracotta with a glass insert) suppress algae growth significantly. Clear glass looks better but algae will appear faster — particularly in bright spots. Many growers settle on a compromise: clear glass for display, water changed weekly to manage the algae.

Water changes: refresh the water every 1-2 weeks. Give the vessel a gentle rinse while you're at it. Old water builds up dissolved minerals, organic matter from any root decay, and algae spores — all of which compound over time. Purified or filtered water is better than tap water because it has lower dissolved mineral content, which means slower buildup of the things you don't want.

Root health diagnostic: healthy passive hydro roots are white or pale cream. If you see roots starting to look brown, slimy, or smelly, that's the early stage of rot — change the water, rinse the vessel, and check the air gap above the waterline.

Feed

This is the step most beginners miss, and it's the one that determines whether your passive hydro plant thrives or just survives.

In soil, plants pull nutrients from the organic matter in the mix, supplemented by whatever you feed them. In water, there are zero baseline nutrients available — whatever you put in the water is exactly what the plant gets. No fertiliser, and the plant slowly starves: weak new growth, smaller leaves, pale colour, eventual decline.

This is where the right fertiliser actually matters, and where passive hydro differs meaningfully from soil:

Most liquid plant fertilisers use urea as their nitrogen source. In soil, that's fine — soil microbes convert urea into ammonium and then nitrate, the forms plants can actually absorb. In water, those microbes aren't present at meaningful concentrations. Urea-based fertilisers in passive hydro break down inefficiently, and what doesn't break down can build up as ammonia, which is toxic to roots.

Aroid Plant Food is built differently. It's urea-free — the nitrogen is already in nitrate form, immediately available to roots without microbial conversion. That makes it the right Plant Runner product for passive hydro, even if your plant isn't strictly an aroid. The chelated micronutrients (iron in particular) also stay available across the pH range passive hydro water tends to drift into.

Dosing for passive hydro: Aroid Plant Food's label rate is 2mL per litre for potted plants. For passive hydro, use half to a quarter of that rate — around 0.5-1mL per litre at every water change. Nutrients in water are far more immediately available than in potting mix — there's no soil buffering, no slow-release effect — so soil-rate concentrations can cause nutrient burn, especially with chelated iron building up over time. Conservative is safer.

Cadence: feed at every water change while the plant is actively growing (spring through autumn). In winter, when growth slows, drop back to feeding once a month at half strength — or skip entirely if the plant is dormant.

Common issues

Algae in the vessel. Inevitable in bright light with clear glass. Switch to an opaque vessel, or accept it as a regular cleaning task. The algae won't kill the plant unless it gets dramatic, but it does compete for nutrients and looks rough.

Brown, slimy roots. Early-stage rot. Change the water, rinse the vessel thoroughly, check that you've got an air gap above the waterline. Trim the worst-affected roots back to white tissue with sterilised scissors. The plant usually recovers if caught early.

Yellowing leaves. Usually one of three things: not enough light, not enough nutrients (haven't been feeding), or not enough water changes (mineral buildup or algae competition). The yellow leaves troubleshooting guide covers the wider diagnosis if your plant is in soil; for passive hydro the diagnostic order is light → feed → water freshness.

Slow growth. Often genuine — passive hydro plants do grow more slowly than the same plants in soil, particularly in winter or low light. If your plant is pushing some new growth and looking healthy, slow is fine. If it's not pushing growth at all, work through light and feed.

Pests. Less common in passive hydro than in soil — no fungus gnats, fewer issues with soil-borne pests. Spider mites can still show up on the foliage in dry indoor air. Wipe leaves with a damp cloth or Neem Oil Leaf Shine to keep the foliage clean and spot problems early.

Getting started

The simplest possible setup:

1. Take a cutting from an existing plant with at least one node (the bump where a leaf and aerial root emerge from the stem). Or rinse the soil off the roots of an established plant if you want to transition it.
2. Choose a vessel. Mason jars, glass bottles, ceramic vases — whatever holds water and looks good. Opaque is more forgiving than clear.
3. Fill the vessel one-third to halfway with filtered or purified water. Add Aroid Plant Food at 0.5-1mL per litre.
4. Place in bright indirect light. Not direct sun.
5. Change the water every 1-2 weeks and re-add fertiliser at the same rate.

That's the whole routine. No extra steps, no daily checks, no complicated equipment.