QotW: When's the best time to water my plants?

Phil Sansom dug out the answer by asking Anthony Bridgen from Cambridge University's Sainsbury Laboratory and Guy Barter from RHS...

Kevin - Does watering my veggie garden in the morning mean that water uptake will coincide with the intake of sunlight, and give my vegetables the best chance? Or is it just as good to water in the evening?

Anthony - In many ways it doesn’t matter too much when you water in regards to the happiness of your plants.

Phil - That’s Anthony Bridgen from Cambridge’s Sainsbury Lab. Anthony, thanks for answering this week’s question. Next week’s question comes fro- ...no, hang on Kevin, I’m sorry. That’s not the end of the story - there are a few considerations here. First - when you water might change how much water evaporates away instead of getting to the plant.

Anthony - If it’s the height of summer, you certainly don’t really want to be watering your plants in the middle of the day. A lot of the water we’re applying to the soil will then just evaporate straight off and be lost, and the plant won’t be able to use it. Additionally, their ability to utilise the water that they are getting might be less.

Phil - That’s because in the midday heat - especially in Australia where Kevin lives - plants might shut their pores slightly to lose less water, and might lower their rate of photosynthesis too. There’s a common myth here too about “leaf scorch”, that water magnifies rays of sunlight to burn the leaves - apparently that’s not true, but midday still not a great time to water.

Anthony - Watering in the morning is a good choice because evaporation from the soil will be lower, because temperatures are lower in the morning. Watering in the evening has the same benefits as morning, but make sure you don’t water the foliage in this instance as, if it’s cold, water droplets containing particles can act as nucleation points, causing freezing at warmer temperatures.

Phil - Evaporation from the soil - and frost aren’t the only considerations. Guy Barter, chief horticulturist at the Royal Horticultural Society, points out that plants might run into the most trouble in the heat of the day.

Guy - If plants are short of water they will shut down in the midday heat and sun in summer. Watering in the morning before they get to the midday deficit is effective. It is also worth mentioning, when they shut down the damage has actually been done, so watering in the morning to head off soil moisture deficit is actually a good idea.

Phil - Kevin, morning seems to be your best choice - but don’t beat yourself up if you can’t get up with the sunrise. Guy agrees with Anthony that this is kind of much of a muchness.

Guy - In real life gardeners take advantage of the fact that plants have evolved over millions of years to be extremely good at coping with lack of water. You know that if you miss a watering, plants look a bit sick but they seldom die, they get used to it. Damage is done of course - very difficult to detect damage - but the plant actually lives. Under these circumstances watering in the evening is good enough. You’re only topping up the soil to keep the crop stress to reasonable levels. You may see a bit of wilting in the middle of the day, but no, it’s not a perfect universe, and one has to put up with a certain amount of stress in real life.

Phil - Life lessons for us all, I think. Thanks Guy Barter and Anthony Bridgen. Next time - what’s in a name? We’re answering this question from Beate.

Beate - If three hundred years ago there was one person with a certain surname, how many people could have their surname today?