[Hosta Frequency Kenneth]

What's the story with drainage?

When the shows and books tell you to throw a handful of grit into the hole under Lillies or Tulips, does this really get the water away faster? Does the clay not just clog up the holes between the grit particles?

When Monty Don shovels sand into his compost mix for potting up lemon trees, how does this limit the water retaining properties of the mix? Does it not just reduce the amount of root space?

I saw him buiding an asparagus bed before, he dug out a big hole and shoved loads of grit into it and mixed it with the clay...he needed lots and lots of grit. (and he was using those little plastic bags of grit from the garden centre that cost about 1000% more than they should)

What about mixing sharp sand into a bed to improve drainage? Or the lawn?
According to some is the worst thing you can do is mix sand with clay you are basically making a cement-like mix which is disastrous for growth.

RHS advice on clay here https://www.rhs.org.uk/advice/profile?pid=620

If I have a pot of lavender or some herb that doesn't like too much moisture, if I put a layer of stones and grit at the bottom of the pot am I not just effectively reducing the size of the potting mix? why not just use a shorter pot?

Interesting article here http://puyallup.wsu.edu/wp-content/uploads/sites/403/2015/03/soil-amendments-2.pdf

And what about putting crocks of terracotta into the bottom of the pot to improve drainage? Are pots really that badly designed? Does the compost not flow around the terracotta anyway? I saw a guy on a show before actually smashing a perfectly good pot to make crocks. Surely this is not the most intelligent way to go??

Has anyone any quantifiable evidence about how drainage really works..would be a great experiment. Anyone going for the BT Young Scientist next year???

Your thoughts?

[tagwex]

Not too many youngsters on here.....

[Greengage]

Re darinage for the bulbs some bulbs like Frittillaria imperialis should be placed on a bed of grit this allows water drain away underneath and stop it rotting the bulb also good idea to place it on its side stops water lodgeing between scales also causing rot.
Good quality top soil is made up of clay,silt and sand, see diagram here.
http://www.azimuthproject.org/azimuth/show/Soil
i dont watch monty but im sure its not sand it is probable grit to allow roots air space in the soil. There is a big differance between sand and grit i dont think anyone uses sand for anything in gardening except mixing cement.

[kindredspirit]

"Doctors differ and patients die."

The best thing to do with clay is add old fashioned manure. Loads of it. Sand doesn't help it.

I hear lots of people arguing against a "perched water table" in troughs and pots. I tend to agree with them but still put something in to ensure the drainage hole doesn't get blocked.

You have some good points.

[Hosta Frequency Kenneth]

Thanks for replies

It was definitely sand in the lemon mix, it was on 'Big Dreams Small Spaces' last week. You would often hear advice to use 'sharp sand' as is has larger particles than clay and allows water to pass through. Sharp sand is better than ordinary sand as it is coarser has more faces, or so the received wisdom goes.

RHS lemon mix advice:
"If your pot is on the heavy side add up 20 per cent sharp sand or grit."

And as regards bulbs:
If you put a bed of grit into a hole, what fills the gaps between the particles of grit? If soil fills it then it cancels the effect of grit, and hinders drainage. If air fills the gaps, does this not damage the roots, allowing water to sit in the gaps and rot the roots causing the opposite effect? What about damage from frozen water? What about the usual advice to firm in plants to eliminate all air pockets?

And where does the water go? If it flows past the grit does it not stop flowing again when it meets clay? A french drain works because it moves water away using a long graded trench, but surely a stagnant hole in the ground will only cause water to pool?

Re: perched water table, a few years ago I decided to plant up an old Belfast sink with herbs. All advice was to 'improve drainage'. I spent some time making a lower layer of grit and small stones, covering this with a permeable hessian sack then putting compost on top of this. Sure, the water drained away when it got to the gritty part, but this didn't affect how the water in the upper part behaved, i.e. I think it was a complete waste of time. I think keeping the drainage holes clear is definitely a priority but a whole layer of grit at the bottom of the sink was just a waste of space.

Maybe I'm thinking about this too much..Someday I will do an experiment and report back!