How to Grow Watercress in a Pot Standing in Water

The tray is the stream, so the water has to move

Wild watercress grows in shallow running water, and what running water actually does is simple: it keeps the roots wet, keeps the water cool, and carries away everything the plant and the soil release. At home you cannot make water run, so you replace it. A pot sitting in an inch or two (3 to 5 cm) of water that you tip out and refill every day or two behaves almost exactly like a slow stream.

Water left to stand does the opposite. Within three or four days in warm weather it goes cloudy and smells sour, the roots sitting in it stop taking up water, and the lower stems turn brown and slimy. Nothing about watercress is difficult except this one habit, and everybody who fails at it fails here.

1. Use a solid tray, not a saucer A plant saucer holds half an inch and dries out by lunchtime on a hot day. Use a washing-up bowl, a seed tray with no holes, or any container that holds two inches (5 cm) of water around the base of the pot.
2. Stand the pot in it, do not submerge it Water should come one third of the way up the pot, no higher. The top of the compost must stay above the water so the crowns and the growing tips are in air. A pot sunk to the rim rots at the collar.
3. Tip and refill rather than top up Topping up leaves the old water underneath. Pour the tray out completely into the garden and refill with fresh water. In summer do it daily; in cool weather every second or third day is enough.

If the water in the tray ever smells, you have already left it too long. Tip it out, rinse the outside of the pot, and go back to a shorter interval than the one that produced the smell.

Clean water means tap water, and cold is better than warm

Fill the tray from the tap. Mains water is treated and safe, and watercress is entirely unbothered by chlorine at tap concentrations. What you must not use is water from a butt, a pond, a ditch or a stream, because that is exactly the water that carries the parasite risk this crop is known for. The point of growing it in a tray is that you control what touches the leaves.

Cool water also keeps the plant sweet. Watercress grows best with its roots between 50 and 68F (10 to 20C). On a hot afternoon a shallow tray in the sun can reach 86F (30C), and at that temperature the plant tastes harsh and starts running to flower. Refilling in the evening with cold tap water pulls the root temperature back down, and a tray in light shade from midday onward stays several degrees cooler than one in the open.

Compost, pot size and a lid of gravel

Use a heavy compost that holds water rather than a light peat-free mix that floats apart. A loam-based compost, or a all-purpose potting mix with a third of garden soil mixed in, sits still and does not cloud the tray. Fill the pot to within an inch of the rim and firm it, because loose compost lifts and spills over the sides the first time you fill the tray.

A pot 8 inches (20 cm) across and at least 6 inches (15 cm) deep holds enough compost to keep a clump going for a season. Shallow pots dry from the top even standing in water, since the wet zone only rises so far by capillary action. Finish with half an inch of coarse grit or gravel over the surface: it stops the compost washing out when you water from above, and it keeps the stems that sprawl across the top out of direct contact with wet soil.

1. Firm the compost hard Press it down with both hands until the surface is solid. Watercress roots into a firm medium and stays put. Loose compost turns into sludge within a fortnight of standing in water.
2. Soak the pot before the first tray fill Water the pot thoroughly from the top until it runs out of the base, then stand it in the tray. A dry pot floats and tips over, and dry compost takes days to wet through from below.
3. Top with grit A half inch (1 cm) layer of grit or fine gravel keeps the surface clean, discourages the green algal film that forms on bare wet compost, and stops mosquito larvae finding still soil to sit against.

Light, feeding and keeping the plant bushy

Watercress wants bright light but not a full day of scorching sun. Four to six hours of direct sun in spring and autumn is ideal; in high summer move the tray where it gets morning sun and afternoon shade. Deep shade makes long pale stems with sparse small leaves, which is the other common failure after stale water.

Feeding is light. The plant is a leaf crop growing in a small volume of compost, so a balanced liquid feed at half the strength on the bottle, added to the tray water once a fortnight, keeps the leaves dark and thick. Do not feed heavily. Overfed watercress grows soft, floppy stems and the tray goes green with algae within days because you have fertilized the water as well as the plant.

Pinch the growing tips out once the stems reach 6 inches (15 cm). Every pinch produces two side shoots, and a pot pinched three or four times early in the season carries far more leaf than one left to run up on single stems.

What to write down

Watercress rewards a very short record because its whole management is a rhythm rather than a set of events. Note the date you set the pot up, the position you chose, and then a single running note of how often you actually changed the water in each month. Next year that tells you when daily changes became necessary and when you could ease off.

Also record the date the leaves first turned hot and peppery, and the date the first flower stem appeared. Those two dates, read against the weather, tell you how much summer shade you need to add and whether it is worth starting a fresh pot from cuttings for autumn. One line per month is enough.