Sunday, 12 September 2021

How to water plants in pots - properly

I seem to be always writing about this - but it's been a couple of years since the last article, and just the other day I saw a perfect illustration of the problem, so let's run over it again.

Plants which you buy from garden centres are potted up using compost: not soil, or earth, but compost.

These days, we are all trying to reduce the amount of peat we use in our compost, so most of the plants you buy will be in peat-free, or peat-reduced, commercial compost.  And when I say "commercial compost" I mean the sort of stuff you would buy in a bag from the garden centre, not the compost that you make in your own garden.

Now, the problem with commercial compost is that it does not hold water very well.  And once it dries out, it is very difficult to re-wet it - as you will know if you have found an oldish half-bag of compost in the shed, and have been mystified as to how it has apparently turned into reddish-brown dust, and is now totally useless for potting up. There is a way to deal with it, I wrote about it at length, here.

This also happens to compost with plants growing in it: it's fine, as long as it is regularly watered, and kept moist, but if it is allowed to dry out - oh woe! It becomes almost impossible to re-wet it, and the plants usually die.

Here is an absolute classic, text-book example. My Client had bought five lovely big ferns, but - unknown to me - had completely failed to water them, for the week before my visit.

I cheerfully issued the instruction: "Please give those plants a good drenching before they are planted," and this was done.

But when the holes were dug, and the plants were de-potted (an essential step before planting: take off the plastic pot!! Don't laugh, I've seen it done - or, rather, not done...), this is what we found:


The water has soaked just the top inch or so, and has totally failed to soak all the way through the bone-dry compost.

You can see the trickles, down the sides, where the water was unable to get "in" to the compost, and has run down the sides of the pot, creating a bit of a puddle at the very bottom, but completely missing out on the centre of the plant, which is where you would expect to find the majority of the roots.

So,what's to be done?

Firstly, learn to tell the difference between a pot which has been adequately watered, and one which is bone dry and about to die.

How? Two easy ways to tell, and that's quite apart from simply looking at the upper part of the plant, to see if it looks dull, lack-lustre and is visibly wilting...

The best way is to check how heavy the pot is. Moist compost is heavy, so the pot will feel heavy.

Of course, if your plant is in a permanent, decorative, pot, you won't be able to lift it up easily: and even if you could, the decorative pot will have a weight to it, so you won't be able to tell.

So, we turn to the other method: press the tip of a bare, dry, finger onto the surface of the compost. If your finger comes away clean, then the pot needs water. If there are one or two crumbs of compost sticking to your fingertip, then the pot still has a good amount of moisture in it.

Secondly, if your finger is clean, if the pot weighs nothing, if the leaves are dull and droopy: then the only thing to do is to immerse the entire pot in a bucket of water, and hold it down until it stops bubbling. This can take some time - ten minutes, fifteen minutes - so a brick or two can be useful, to weigh down the pot and hold it underwater.

Once you have successfully re-soaked the compost, lift it out of the bucket and let the excess drain away, then leave it to recover. If the leaves perk up, hooray! you have saved it. If they remain droopy and dull, even a day or so later, then the plant has probably reached what the RHS call, rather delightfully, the PWP or Permanent Wilting Point, ie death. Nothing more can be done, other than to doff our caps, bow our heads, and say a sad farewell as we tip the plants out, and bung it on the compost heap.

Having gone to the effort of saving your plants-in-pots, you can help them to survive future droughts by watering them less often, but more thoroughly.

The rule, if you can call it that, is that it takes a depth of 1" of water above the surface of the compost, to penetrate 8" down. It can be hard to work out if you have given the plant an inch of water, because it will - or, it should - start to soak in, as soon as you start to pour it on. But think about how much water it takes, to cover the surface of any given pot, to a depth of an inch (that's 2.5cm for you youngsters), and aim to gently pour that much water on to each pot.

If you find that the water all runs out of the bottom, then your compost is still not thoroughly wetted in the middle, so try the dunk-and-soak routine again. 

You can also try standing recalcitrant pots on saucers, so that the water will pool at the bottom, instead of running away, and therefore has time to be soaked back up into the pot. But a quick word of warning about this method - very few plants like to sit with their bottom in water all the time, they need air as well as water, so remember to empty the saucers after an hour or so, if they still contain water. 

After a while, you will get a "feel" for how much water your pots need, and hopefully you won't ever find another pot full of bone dry, powdery compost-dust!


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