Why pots and baskets stop taking water, and how to fix it

Wetting agents · part seven

By Joe, Founder of Dr Forest · August 2026 · 16-minute read

Retail growing media in the UK is peat-free now, and the components in the bag rewet at very different speeds. That is where this question gets interesting.

A wetting agent for hanging baskets and pots earns its keep on compost that has already dried out, and does almost nothing on a bag that is still properly damp. The container research is unusually specific about where that line sits. Peat dried to 40% moisture by weight took up only 14% of its container capacity on the first watering, and still only 20% after three [1]. A compost wetting agent changes how much of the water you apply the material will actually accept. It cannot do anything for compost that is wet enough to accept water on its own.

The mechanism, surface tension and contact angle and what a surfactant changes about both, is covered in the guide to what a soil wetting agent is. This page stays on the container question: what the published data says about pots, baskets and peat-free mixes, and whether the product is worth your money in one. Grass is a different job, and it has its own guide to using a lawn wetting agent.

Why compost stops accepting water when it dries

Growing media contain water-repellent material as standard. Nemati and Jeong put it plainly: waxes, resins, organic acids and other chemicals present in organic media components, principally peat moss and bark, are inherently water repellent [7]. Wet, they cause no trouble. Dry, they dominate the surface the water has to cross.

Two thresholds run through this literature. At around 40% moisture by weight, peats become hydrophobic with a very low ability to rewet. At 60% by weight, every material tested behaved as hydrophilic [1]. The physical boundary sits at a contact angle of 90°, above which capillary rise stops. Durand and colleagues measured milled white peat at 90.0° at both 40% and 50% moisture, falling to 87.0° at 60%, whilst wood fibre held between 85.5° and 86.1° at every moisture tested. Read those 90.0° values as at or above 90°, since the capillary-rise method cannot resolve angles beyond that.

Now the honest part. Drying is not a clean round trip. Michel, Rivière and Bellon-Fontaine took decomposed horticultural peats through drying and rewetting cycles and found that the more decomposed the material, the greater and the more irreversible its shrinkage [2]. Michel's single-authored review reports hysteresis across those cycles and ranks repellency on desiccation from highly decomposed peat, through bark, weakly decomposed peat and wood products, to coir [3]. A pot dried hard does not simply return to where it started, and no additive puts back a structure that has collapsed.

Where the water goes when you water a dry pot

This is the number I would want if I were deciding whether to bother. Surface watering a dried-out pot mostly fails, and the failure has been quantified more than once.

Peat at 40% moisture held 14% of its container capacity after one irrigation and 20% after three [1]. At 50% moisture the same peat reached 0.36 after one and 0.60 after three; at 60% it reached 0.77 and 0.85. Schulker and colleagues, working with sphagnum peat, coir and aged pine bark, found peat at 33% initial moisture captured around 42% of container capacity at its best, whilst pine bark managed around 81% under five-minute pulses at the same moisture [4]. Peat's container capacity in that work ranged from 15.2% to 84.1% by volume depending on how it had been conditioned, against around 10% variation for coir and pine bark.

So when you tip a can over a shrunken rootball and the water appears at the drainage holes in four seconds, that is not the pot being full. Most of it went down the gap between compost and pot wall, and out.

Peat-free compost, component by component

Bagged retail growing media in the UK is peat-free, so the question becomes how wood fibre, bark and coir behave. Wood fibre comes out well. It stayed hydrophilic throughout and reached 0.88 to 0.91 of container capacity on the first irrigation at every moisture content tested, rising to 0.96 by the third [1]. The catch is what it costs elsewhere: 60% by volume was needed to bring a peat blend close to wetting-agent-treated peat, and water holding capacity fell from 75% by volume for peat to 49% for that blend and 29% for wood fibre alone. A systematic review puts easily available water at 33.5% by volume for peat, 25.1% for a 50:50 wood fibre and peat mix and 13.8% for pure wood fibre, and calls 30% to 50% substitution optimal [9]. That is review-level evidence.

Coir deserves a correction. It is sold on wettability, and at 50% initial moisture it is superb: an initial hydration index of 0.98 to 1.0, full hydration in a single event [5]. Let it dry to 25% and the index collapses to 0.14 to 0.45, and with no wetting agent or a low rate it failed to reach container capacity even after ten hydrations. Peat at the same 25% ran 0.13 to 0.34. The authors' own summary is that initial moisture was the predominant effect. Schulker and colleagues found coir among the most stable materials they tested, and both results stand, because the moisture levels tested were different. Dry coir is not the forgiving material the bag suggests.

One gap, stated plainly. I found no published trial of UK retail bagged peat-free compost and how it rewets. The RHS peat-free page says only that watering and feeding requirements may differ and to follow the advice on the packaging [10]. It makes no claim that peat-free is harder to rewet, and it does not mention wetting agents at all.

Do wetting agents actually work in a container?

Yes, measurably, at moderate moisture. Fonteno, Fields and Jackson ran coir, peat and pine bark with and without AquaGro-L at 187 mL per cubic metre, four replications, ten hydration events each [6]. At 45% moisture with the wetting agent, first-irrigation hydration efficiency was 0.95 for coir, 0.76 for peat and 0.99 for pine bark, all reaching 1.00 by the third event, with maximum hydration of 74.1% by volume for coir, 70.4% for peat and 38.0% for pine bark. Untreated peat at that same 45% managed 0.23 against 0.76 treated, which is the strongest single number in favour of the product.

The same paper carries a partial null, and it matters more than the headline. At 30% moisture, peat gave a first-irrigation efficiency of 0.19 with the wetting agent and 0.19 without. Coir improved from 0.29 to 0.43. And peat at 15% moisture with a wetting agent (0.15) still did worse on the first irrigation than peat at 45% with none (0.23). Starting moisture beats the additive, and no wetting agent rescues a rootball that has gone properly dry in one go. This is conference proceedings, a lower tier than a journal paper.

The counter-evidence, given its weight

Nemati and Jeong tested three commercial wetting agents at seven rates, 0, 1, 2, 3, 5, 10 and 20 times the recommended dose, in 100% sphagnum peat, using Impatiens and Antirrhinum majus raised from seed and Calibrachoa and New Guinea impatiens from rooted cuttings [7]. Their finding, verbatim: adverse impacts of wetting agents occurred at concentrations between 3 and 10 times the recommended dose, depending on wetting agent type, plant type and plant growth stages. They add that very little attention has been paid to those adverse impacts.

Three times the label rate is not a wide margin. It is one heavy-handed glug in a two-litre can. Growth stage mattered too, which is why seedlings get a lower dose in the table below. The saponin content of our own concentrate is not published, so I will not give you an in-use figure in milligrams per litre; apply at label rate and do not over-concentrate. One caveat on the whole body of work above: nearly all of it comes from two research groups, at NC State and at Institut Agro Angers, often collaborating. That is a narrow base for a confident claim.

Do you need one in fresh compost?

Mostly no, and I would rather say so than sell you a bottle you will not use. All three commercial mixes in the Fields trial reached full hydration on the first application when they were at 50% initial moisture [5]. Dried to 25% they diverged badly: one reached container capacity in three events, the other two never got there in ten. Fresh, correctly moist bagged compost is not the problem. Compost left to dry out is.

Whether the mixes on UK shelves already carry a wetting agent as supplied, I cannot tell you. The trials do not state it and I found no published survey of retail bags. What the evidence supports is keeping compost above the moisture where it starts to refuse water, rather than adding a surfactant to a bag that is already behaving.

Rewetting a pot that has gone bone dry

Longer watering does not fix it. Across every substrate Schulker and colleagues tested, around 70% of the water taken up during a 60-minute pulse was taken up in the first five minutes [4]. Increasing the depth applied helped a little, raising hydration by 6% to 8%. Repeated events are what move the needle: peat at 50% moisture went from 0.36 after one irrigation to 0.60 after three [1]. Several separate waterings a few minutes apart, then, rather than one long soak.

How many of them, I can only give you as craft rather than as a trial result: a container that has dried right out usually takes two or three waterings with the solution before it wets evenly again, then plain water until it next dries out hard.

Immersion is the other lever. Michel and colleagues compared surface drip against sub-irrigation directly and concluded that rewetting properties depend on the wettability of the material, whilst the dynamics of water uptake are governed by the irrigation method [8]. Standing a pot in water changes the shape of the uptake curve rather than overriding the material. It still helps, because the water moves in from below instead of racing down the sides.

How long to stand it there, I cannot cite. No source I can verify states an immersion duration, and the widely repeated twenty to thirty minutes traces back to nobody. The RHS says only to dunk small pots into a bucket or tub of water, with no duration attached [11]. As craft rather than evidence: leave it until the air stops bubbling out, which you can at least observe. Adding our Natural Wetting Agent to that bucket at root drench rate, 0.5 to 1 teaspoon per litre, is where it does its most useful work.

Hanging baskets

Baskets dry faster than anything else in a garden, and they are the container most likely to fall below the moisture where compost stops accepting water. The only container-specific UK data I could verify comes from RHS Science, led by Tijana Blanusa at the University of Reading [12]. Working with petunias and busy lizzies, the team found it took 160 ml of water a day to saturate the compost supporting each petunia, whilst 80 ml was enough to grow a good plant. Irrigation applied 5 cm below the surface through porous hose improved plant quality even though the upper compost was dust dry, and over-watering produced lower-quality plants.

Do flag the attribution: that page carries no publication date, no journal reference and no co-author list, so it is RHS Science web content attributed to Blanusa rather than a peer-reviewed paper. No published measurement exists of how much water bypasses a hanging basket specifically. Anyone quoting a percentage for that has made it up.

What to do, by container

Rates come from the label. A teaspoon is 5 ml, so 1 teaspoon per litre is 0.5% by volume, and the measurement calculator handles the rest. If you want the reasoning behind each of those rates, the rates and coverage guide sets them out by job, and the piece on choosing a wetting agent covers what separates the products on the shelf. Short answers to the questions that come up most often sit in the wetting agent FAQ.

Final word

Lift a pot that has been standing in August sun and you can feel the whole thing has gone light. Pour a can over it and most of that water is at the saucer before you have finished pouring. Fourteen per cent of capacity on the first go, twenty after three. Get the basket into a bucket, put the wetting agent in the bucket, and give it four or five goes rather than one heroic one.

Frequently asked questions