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I asked this question about my potted lilly pillys. I have another separate question.

I live in a temperate climate zone. I have plants on a east facing balcony- they get sun from about sunrise till the middle of the day.

I have Syzygium smithii, lilly pillys native to Australia, planted in square concrete planters. They are approx 2.5 foot wide and deep, and 2.5 foot tall.

They are unpainted (I had wanted to paint them before they were planted and was talked out of it).. they dry out very quickly.

Would painting the outside help conserve water, and if, so would an oil based paint be the best?

Would it have been better to paint them on the inside?

What sort of material is good to retain water, ie terracotta, concrete?

liyylpilly

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Hmm... I'm not sure water can flow through concrete. I found a study titled "Permeability study of cracked concrete" which says:

Most researchers believe that a well designed and manufactured concrete is originally water-tight, containing discontinuous pores and microcracks.

Although concrete seems porous at first, the pores actually don't connect with other pores or go all the way through the concrete, so the concrete won't pass water unless it is cracked. If there is a crack, then paint isn't what's needed... you need a crack repair product.

You can still paint it for asthetic reasons if you want. As far as which paint type to go with, I'd let the guys on the home improvement site make a recommendation https://diy.stackexchange.com/ It appears from a google search that water-based latex and oil are acceptable for concrete, as well as polyurethane or epoxies. Whichever you choose, make sure it's at least "exterior" grade paint.

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Theoretically, painting the pots with reflective white paint, could reduce the heat absorbed from the sun, and thus the evaporated water.

Who knows if the practical effect would be measurable. You could try calculating it or asking the guys over at Physics.SE.

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  • That's interesting. I didn't think of that.
    – Randy
    Commented Sep 4, 2013 at 15:48
  • I think you'd see a big difference between black paint and bright white paint, though. But yes the difference between natural pale grey and white might be minimal.
    – winwaed
    Commented Sep 5, 2013 at 13:01
  • @winwaed, very possible. Though, there will be one more difference - concrete is rough, while paint is smooth.
    – Vorac
    Commented Sep 5, 2013 at 14:25
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You can put more clay in the soil composition: it retains a lot of water.

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    Can you expand on this a bit more? What proportion of clay to organic matter do you recommend, will any kind of clay work?
    – kevinskio ♦
    Commented Sep 4, 2013 at 17:29
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    Also perlite or vermiculite could have similar effect. Perlite is insanely cheap, while vermiculite is said to be the best of the best for water retention. These solution sill not hinter air circulation as much as clay would. Just guessing, though!
    – Vorac
    Commented Sep 5, 2013 at 7:48

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