Saturday, 17 March 2012

Watering terracotta pots - not such a problem as you might think

My apologies for the length of this post, but I wanted to tackle this topic thoroughly...

Now that hosepipe bans are certain for many areas of the country from next month I can see that panic about terracotta pots may spread. I am tired of reading articles which baldly state that terracotta dries out much more quickly than plastic and therefore should be avoided. Time to stick my oar in.

Terracotta terror is unjustified
July 2008 Happy plants in terracotta in our hot, dry stockyard.
Not as difficult as you may think
Visitors to the pottery sometimes assume that I spend all my time watering the 300-500 pots on display. In a typical summer I water 1-3 times a week, depending on the weather. After all I'm only there three days a week and I have plenty of other tasks! I do ask Babs to water at the weekend sometimes for me.
It takes about two hours to water the displays with water running at watering can speed from the hose. Some plantings may need an occasional top up from a watering can. I have about 200 pots at home and usually use a watering can with rainwater or water from the kitchen sink - it's manageable...

In my view it is not the pot material that makes the difference to the quantity of water needed, but the establishment and maintenance of correctly chosen plants.

An established planting in a high-quality terracotta pot does not dry out perceptibly more quickly than in plastic. In a plastic pot the sun heats the root ball more than in cool, porous, insulating terracotta and so it could be argued that more root damage and evaporation may occur. I have not tested this scientifically, but I have been working at Whichford Pottery for eleven years and was obsessed by gardening long before that: at the beginning of each summer I plant hundreds of pots and yes I use plastic at home (being a plantaholic I use anything) as well as Whichford seconds and I can truthfully say that I find the terracotta easier to look after.

When a planting has become well established the fine roots of the plants bind themselves to Whichford terracotta so that if the potful is allowed to dry out at all the rootball does not shrink away from the pot. In plastic the rootball shrinks and a gap is left all around the container so that most water arriving from above runs uselessly down the crack instead of soaking in to the compost.

Lining terracotta with  plastic prevents the roots from binding on, denies the roots oxygen and can interfere with drainage - please don't!
I believe that the vast majority of water lost from an established pot is transpired from the foliage and relatively little is lost through the walls of the pot. Before plants are established and there is un-colonised compost in the pot it does more harm than good to have excess water sitting in the compost, so even in a warm summer good drainage (as in Whichford pots) is very important. Good drainage is sometimes confused with "drying out".
Plastic pots which are allowed to become somewhat dry are also likely to blow over and break precious plants.

Here are my tips for surviving a water shortage with a fine display of pots:

Location, location
Be aware of your garden's microclimates. Remember that pots, even big ones, are moveable! If watering is going to be tricky this year it makes sense to revise your usual display sites and if possible locate most of your pots in at least part shade, preferably keeping them out of the sun in the hottest part of the day (early afternoon). Put them within easy trudging distance of your water butts.
Put the right plants in the right location - sunlovers in sun, but leafy water-lovers with more shade. I'll go into plant choice in more detail next week.
Transpiration (water loss from leaves) rates go up dramatically in windy positions, so keep pots away from exposed sites if possible; bear in mind that walls can look sheltering but may create damaging turbulence and surprising dryness, especially near corners. Use semi-permeable barriers to reduce this effect - trellis, willow obelisks or woven screens, even garden benches can help.
Be aware that a shade-giving fence or house wall may also create a rain-shadow, so that a pot at the foot of the wall receives less of the precious wet stuff.
June 2009 Hostas do very well in Whichford Pots
Here they enjoy the shade on the northern side of the Octagon
but I have to remember that here they are sheltered from rain as well as hot sun
May 2010 One I got wrong. By the time the Humulus lupulus 'Aureus'
had covered this obelisk there was too much foliage to be supported
by such a small pot. I had to keep watering it, even after
moving it from a much windier, sunnier location.
Choose the biggest pots you can afford. The bigger the pot, the easier it is to look after - small pots have a much greater surface area to volume ratio and so will dry more quickly, they are also more fiddly to water. 
When you plant a pot bear in mind the eventual size of the plants. My rule of thumb is that a certain volume of compost/roots will happily support up to about twice that volume of greenery, experience teaches you to vary this according to site and plant choice. It's particularly easy to get it wrong with vigorous climbers.

Most of our really big pots have a base ring which can be taken out so that large plants, trees etc can root through into the ground below to find more moisture. Bunny Guinness often recommends this. Of course you need to be very sure where you want your pot to be!

Group mentality
Clustering pots together creates a more humid microclimate around the display, slowing transpiration; they shelter each other from sun and wind and dry out much less quickly than a pot by itself does.
As the plants expand you can move them slightly further apart. A group is easier to water and has much greater visual impact too.
Sept 2007 This group faces south-west but is still luscious in late summer, helped by its own microclimate

Compost considerations
I use a compost which is a mixture of peat, loam and grit. One feature of loam content is that it makes the compost easier to re-wet and the clay in the loam will hold on to moisture and nutrients. You can add water-retaining polymer granules but I don't because I like to keep many of my plants for overwintering and I find that the granules increase the likelihood of rotting in the winter.
These well-established plants (ALL in pots)
are on a hot,windy corner but are not overfed
and are surprisingly tough. July 2008
I add a little slow-release fertiliser so that I won't have to use soluble fertiliser. I don't feed my plants much because I want them to be tough - I don't need them to be enormous and floppy, I want them to be strong, healthy and fairly compact, needing less water.
I keep watering to a minimum before the plants are planted in the pots too (without allowing them to dry out completely), and have always thought a little neglect now and then toughens plants up - it seems that someone agrees with me (click here to see an article on the possibility of training your plants to drink less)

Give them a good start
Fast establishment of summer plants is key. Thoroughly water the plants you are going to use, then leave them to stand for a couple of hours before you start. This means that the roots slide nicely out of the plastic without being damaged, and the root balls are soft but not waterlogged, so that the roots can carry on growing outwards to colonise the fresh compost without check. If you plant dry rootballs and water them in, much of that water will bypass the hard, dry rootballs and drain through the damp compost.
Make sure that all spaces between the plants are well filled in with compost so that there are no gaps and the rootballs are all covered, then water thoroughly immediately and check that the roots are still thoroughly covered. The surface of the planting should end up about 2cm/1" below the rim of the pot so that there is room to water without washing the compost out of the pot.

Sept 2009 The base of a standard holly tree, mulched with gravel and stones
Mulchy, mulchy
If there is quite a lot of bare compost still to be seen when you have finished planting, a mulch of gravel, pebbles, grit, slate, glass nuggets etc will help to slow evaporation and will stop the surface of the compost from forming a hard cap which prevents water from penetrating easily.
I plant summer pots pretty densely, so that little compost is visible, the plants then knit together and shade the surface and ultimately the sides of the pot. Watch out for large-leaved plants covering the surface of the pot - you may need to water even after rain as they may act as an umbrella.

A proper drink
If your water authority allows it a drip irrigation system is really useful and economic. Ours has been ruined by dogs and utilities excavations, so currently I use the hose and watering cans. Make sure that enough drippers are placed in each pot to avoid dry patches. For the summer set the timer to work in the evening so that the water can soak throughout the potful and be available for all the plants before it starts to evaporate.

Whether you use a watering-can, a hose or an irrigation system the principle is the same. Give a good soak as infrequently as possible at a cool time of day. Do not allow pots to dry out between waterings as there will be run-off because a really dry rootball is hard to re-wet.
Do not water little and often as this will not encourage the roots to colonise all the available volume of compost and will encourage soft and unsustainable top growth. I count (to 60 for a medium-sized pot, 100 for a big one etc) to stop myself from moving on too quickly, and I water at several different points in larger pots.

I never use a spray or a rose, I simply have the water pouring gently and steadily so that the compost is not blasted out of the pot. I sometimes use my thumb to spread the jet slightly if I'm using a hose, the aim is not to wet the foliage and flowers but to get as much water as possible to soak in around the plant roots.

To test whether a planting is dry do not just look at the surface. Observe the plants - are they turgid and happy looking or are they starting to wilt? Stick a finger right into the compost to assess its moisture content.
The Echium pininana in the centre of this picture is looking quite happy...
Sept 2008
But the Echium pininana here is beginning to look
sad - I may have to water today
July 2010
If you observe the state of your compost and plants regularly you will soon learn the warning signals. I use some plants as miner's canaries - I find Echium pininana and Salvia confertiflora are the quickest to show signs of distress, so if they have started to look sad it is time to get watering. This way you can avoid other plants get stressed and keep them flowering and free of powdery mildew.

Paradoxically, if a summer shower is predicted after a dry spell it may be worth doing a light water beforehand to make the compost surface more receptive and to make the plants turgid enough not to get broken by the downpour.

So please don't let the prospect of a hosepipe ban stop you from enjoying your container gardening - look on it as a challenge which will make us think more carefully about making the most effective use of any water we do use!


  1. Some great tips and info here, Harriet. Thank you!

    1. Thank you, Fernando and Joe, I'm so glad people are finding this useful.

  2. I've got loads of terracotta pots - including a fair few Whichford ones and I don't find them drying out a problem. In particular, as you say, I don't get that shrinking back from the side of the pot when they have got dried out as you do with plastic ones. All my hostas are 'potted' and do extremely well.

    Your pictures of pot displays have made me feel excited at the prospect of planting stuff up again this year. I had never thought of planting sempervivums around the roots of my potted trees - they look really pretty.

    1. Thank you Arabella, here's to lots more successful potting! Yes, I've enjoyed looking through some summery pictures too, I love Spring but Summer is the season I ache for...

  3. Great post, Harriet. Good to see some myths about terracotta soundly dealt with :)

    1. Thank you, it infuriates me that the myths get propagated by reputable writers. People have been growing plants in terracotta for a good couple of thousand years - can't be that impractical!

  4. Brilliantly informative post Harriet - I enjoyed it very much.

    Wonderful to hear your thoughts on watering pots, and hundreds of them too.

    Ann-Marie Powell

    1. Thank you, Anne-Marie, I reckon I've earned the right to pontificate about potting now!

  5. Hi Harriet,

    I really learned a lot today through your post! You gave us many excellent advices that I will be happy to propagate!