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Gardens Blog

Gone potty

Watering Our Plants...

Spring is sprung, the leaves and buds are appearing and a gardener’s mind turns to thoughts of pots and containers and the delightful question of what to plant this year. Shall we be tasteful and have our pots all the same or will they turn out like a bag of dolly mixtures? Each to their own and I dare say we will see a selection of each over the coming months.

But beneath the bright and the beautiful, deep amongst the John Innes lies a vexed question: what sits at the bottom of your pot? I have been doing a little on-line research into the truth behind the tradition of ‘a few crocks at the base’ and there are conflicting views on this. The RHS, BBC Gardener’s World experts and much of the literature would have us believe that the first commandment in successful container planting is ‘Thou shalt add that which aideth drainage’. And so we do, obediently bashing up old clay pots, breaking up polystyrene and dumping it into the base.

But light is dawning and I for one am abandoning this practice, based in part on an article by Ken Thompson in the Daily Telegraph in 2013 (how I love the internet). His premise is that when you have a fine layer (i.e. compost) on top of a coarse layer (the crocks) strange things can happen with drainage, particularly if the coarse layer is not totally flat. For example, water can flow off sideways and end up somewhere unintentional. Apparently this is known to hydrologists as ‘textural discontinuity’ and can lead to ‘funnelled flow’ This is bad enough for irrigation but if the water also contains a soluble fertiliser, then your plants are going to be unevenly nourished with predictable results.

Ken Thompson goes on to write “Because it resists compaction and provides good drainage, sand is the basis of most modern golf course putting greens. But the downside of sand is that it holds little water, dries out rapidly and needs a lot of watering. The most popular solution to this problem is around 300mm of sand over a 100mm layer of gravel. Capillary forces within the sand mean that water is unwilling to cross from the (relatively fine) sand to the (much coarser) gravel, creating what hydrologists and geologists call a “perched” water table, essentially one that is higher up than it should be, and above the “real” water table.

Maybe you’re now starting to see the parallel between the sand and gravel beneath a putting green and the compost and crocks in your plant pot. Both are a fine layer over a coarse layer. But the former is designed to reduce water loss from the fine layer and keep it wetter than it would otherwise be, while the latter, if we believe the gardening books, is to improve drainage and keep the fine layer drier. They can’t both be right, although in a sense they are. During heavy rain, the putting-green sand layer eventually becomes saturated, gravity overcomes capillary forces and the water has nowhere else to go but into the gravel, where it drains away rapidly. So the sand/gravel sandwich is well-drained. But once the surplus water has drained away, the sand remains wetter than it would be if it were just sitting on more sand.

Exactly the same happens in your plant pot. When you pour enough water in the top of the pot to saturate the compost, gravity overcomes the capillary barrier at the compost/crocks boundary and it drains away through the crocks and out of the drainage hole. But it would do exactly the same if the crocks weren’t there, and when you stop watering, you’re left with a perched water table in either case, crocks or no crocks. The only difference is that if there’s a layer of crocks, the water table is perched at the compost/crocks boundary, and if there isn’t, it’s at the bottom of the pot.”

And so, all we are doing when we put our layer of shards into the pot is reducing the amount of growing space available to the roots of our beloved plants. If we add drainage material to the compost then we simply hasten the process.

All containers should be drilled to ensure that the plants are not sitting wetly in a sump and perhaps subconsciously we think the crocks stop the soil washing out when we water. This is quite true, but perhaps instead of several inches of old terracotta we could simply place a coffee filter over the holes at the base of the pot. This will contain the soil effectively while allowing excess water to pass through and hopefully all our plants, whether they be gardenias or geraniums, will thrive and blossom right through to the frosts.