Why is my peat-free compost dry on top and soggy underneath?
Why wood fibre and bark dry pale on the surface while the middle of the pot is still wet, and the watering change that fixes it.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 4 min
- Safety
- caution
Short answer
The pale dry crust is not a signal that the pot needs water. Peat-free composts are coarse and fibrous, so the surface dries within hours while the core stays wet, and once the surface is dry it repels water and sends it down the gap at the side of the rootball. Judge by the weight of the pot, and when you do water, water slowly two or three times over rather than once.
Almost everyone who says peat-free compost does not work is watering it as though it were peat. Peat holds water like a sponge and looks damp until it is nearly empty. Wood fibre, bark and coir hold water in a coarser, more open structure that gives a lot of it up at the surface very quickly. The material is fine; the visual cue people have used for thirty years has stopped being reliable.
Safety
What you'll need
- A tray or bucket large enough to stand the pot in
- A watering can with a fine rose (optional)
- Gravel or bark for a surface mulch (optional)
Step by step
- Stop using the surface as your test.The top 20mm of a peat-free mix goes pale and crumbly within a few hours of watering. It tells you about the last few hours of weather, not about the rootball.
- Lift the pot instead.Weight is the reliable test. Water a pot thoroughly, feel how heavy it is, and compare. After a week you will judge it without thinking about it.
- Or push a finger in to the second knuckle.Damp at 50mm means leave it. A bamboo cane or a wooden skewer pushed to the bottom and pulled out works the same way and reads the base of the pot, which is where the water actually collects.
- Water slowly, and go over it more than once.A single fast fill runs straight down the sides. Wet the surface, wait a minute for it to take, then water again, and again. The second and third passes are the ones that reach the middle.
- Rescue a rootball that has gone water-repellent.If water runs out of the base within seconds, the compost is dry, not wet. Stand the pot in a bucket or tray of water up to about half its depth for twenty to thirty minutes until the surface darkens, then take it out and let it drain.
- Close the shrinkage gap.A dried rootball shrinks away from the pot wall and every drop of water then runs down that channel. After soaking, firm fresh compost into the gap with your fingers so water has to go through the rootball rather than round it.
- Add loam to the mix next time.About a third of loam-based John Innes mixed into a peat-free multipurpose transforms its water behaviour: it holds more, releases it more evenly, and rewets far more readily if it does dry out.
- Mulch the surface of the pot.Twenty-five millimetres of gravel, slate or bark stops the surface capping and drying, cuts evaporation and stops splash. It also removes the misleading visual cue entirely.
- Feed earlier than you would with peat.Wood fibre uses up nitrogen as it breaks down, so peat-free pots run hungry sooner. Most bags carry four to six weeks of feed; start liquid feeding at about that point rather than waiting for the plant to look poor.
- Empty the saucer in autumn.A saucer holding a reserve is useful in a July heatwave. From October it keeps the base permanently saturated, which is exactly the condition that rots roots over winter.
Tips
- Weight, not colour. That is the whole adjustment.
- Water twice with a pause between, rather than once for twice as long.
- A bamboo cane pushed to the bottom of a big pot and pulled out is a free moisture meter and more honest than most bought ones.
Common mistakes
- Watering whenever the surface looks dry — In a peat-free mix that is several times a day in summer, and it produces exactly the state people complain about — a dry crust, a dry core and a waterlogged base.
- Assuming water running from the base means the pot is full — It usually means the opposite. Water running through in seconds is water bypassing a dry rootball down the shrinkage gap.
- Buying more compost rather than changing the watering — Every peat-free product behaves broadly this way. Adding loam and changing when you water fixes it; changing brand mostly does not.
If it doesn't work
Surface bone dry, base soaking
Cause: Watering little and often, coarse open mix — Fix: Water thoroughly and much less frequently, judging by pot weight.
Water runs straight through in seconds
Cause: Rootball dried and now repelling water — Fix: Stand the pot in water for half an hour, then firm compost into the gap at the wall.
Plant wilting although the compost is wet
Cause: Roots dying in a saturated base — Fix: Check the drainage hole is clear, remove any saucer, and repot into fresh mix if the roots smell sour.
Growth pale and slow from midsummer
Cause: Built-in feed exhausted and nitrogen tied up by wood fibre — Fix: Start a regular balanced liquid feed at the rate on the label.
Questions people ask
Is peat-free compost worse than peat?
It grows plants perfectly well, but it holds and releases water differently, so the habits people built around peat mislead them. Add loam, judge by weight, water thoroughly and less often, and feed a little earlier.
How often should I water a peat-free pot?
There is no schedule that works, because it depends on the pot, the plant, the position and the week's weather. Lift the pot. That takes two seconds and is right every time.
Do water-retaining gels help?
Modestly, in hanging baskets and small pots. They are not a substitute for enough compost volume or for loam in the mix, and they do nothing at all for a pot that is solid with roots.