Why potting mix stops soaking up water, and how to fix it
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If you tip a watering can over a pot and the water runs straight down the inside and out the bottom in about 3 seconds, the mix hasn't actually taken any of it up, and the plant is still dry even though you've just watered it. That's the commonest reason a potted plant dies of thirst in a pot that looks like it gets watered every day.
What's actually going on
Potting mix is mostly organic matter, being composted bark and coir and peat, and when that material dries out completely it turns water repellent. The surface of each particle goes waxy, so instead of soaking in, the water beads up and looks for the quickest way out, which is usually the gap between the mix and the side of the pot. The mix in the middle can stay bone dry through weeks of daily watering.
You'll know it's happening because the pot stays light when you lift it, and because the water comes out of the drainage holes almost straight away rather than a minute or so later.
How to bring a dried out pot back
Standing the pot in a tub or a deep saucer of water for 20 minutes is the fix, because soaking it from the bottom gives the mix time to take water up slowly instead of shedding it. Once it's heavy again, lift it out and let it drain properly. Don't leave it standing in water after that.
If the mix has shrunk away from the side of the pot, push it gently back against the edge first, otherwise the water just runs down that channel again and you've achieved nothing.
A wetting agent is the other half of it
A soil wetting agent breaks the surface tension so water can get into the mix rather than sit on top of it, and honestly it's worth using on anything in a pot over summer. We stock Brunnings Easy Wetta Crystals in a 500g pack in the shop. You scatter them over the surface and water them in, and you'll want to do it again later in the season, because they don't last forever.
Stopping it happening again
The thing that causes it is letting the mix go completely dry, so the honest answer is to not let that happen, and a layer of mulch over the surface of the pot is what buys you the margin. Mulch slows the evaporation off the top, so the mix underneath stays damp for longer and never gets to the point where it starts repelling water.
Watering less often and more deeply beats a daily splash, because a splash only ever wets the top couple of centimetres and the roots down the bottom never see any of it.
The pebbles myth, while we're here
Putting a layer of pebbles in the bottom of a pot doesn't improve drainage, and it's worth saying plainly because it's written just about everywhere. Water doesn't move easily from a fine material into a coarse one, so it actually sits in the mix just above the pebbles instead of draining away, and all you've really done is make the pot smaller. A pot with a drainage hole and a decent mix doesn't need anything in the bottom of it.
Repotting
If the roots are coming out the bottom then it's time to move up a size, and about 2 fingers wider than the old pot is right, because jumping straight into an enormous pot leaves a lot of wet mix that no roots are using. Water the plant while it's still in the old pot so the roots aren't stressed when you move it, and let it drain before you tip it out. Then tease the outside roots loose a bit so they'll grow out into the new mix rather than keep circling round.
Shop related products
- Brunnings Potting Mix 70L. A general potting mix for pots and tubs.
- Easy Wetta Garden Mulch 50L. Goes over the surface to slow the mix drying out.
- Brunnings Vermiculite 5L. Holds moisture in a mix, handy for seed raising and for pots.
- Brunnings Garden Compost with Seaweed 25L. Organic matter for garden beds rather than pots.
Did you know we also stock Brunnings Easy Wetta Crystals 500g, the Easy Wetta Mulch Block 60L and gardening gloves in the shop. They're not all on the website, so give us a ring on (08) 8536 4333 to check what's on the shelf.
The rest of it is in our garden centre, and you can have a look at the garden range online any time.
Read next
- Choosing a potting mix, and what the ticks on the bag mean
- How to improve sandy soil so it holds water
- How to fix uneven watering in your garden
I still manage to let a pot dry right out on the back step now and then, so this is advice I give myself as much as anybody else.
Cheers,
Candeece
