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I've heard that you shouldn't water mid day, but should you water more during the day, or night?

What is the rule of thumb on what part of the plant you want to affect more?

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In my experience, watering at night allows the water to penetrate deeper, but it does promote slugs,snails, mildew and mold etc. During winter the water can also freeze on the plants and damage them. (much more than frost would).

I think that watering early in the morning is the best (like 4am) - avoids the freezing problem, and in my experience can actually "melt" frost. If you are still watering in early Winter/Autumn.

Watering during the day has the benefit of cooling the plants off when it's SUPER hot. But it really wastes a lot since most of it will evaporate and won't penetrate the ground. If you are lucky enough to have a greenhouse - I think this is the best time to water, because you cool the greenhouse down and you get nice humidity ( i.e. foliar irrigation )

If I had an endless amount of water .. I would water really well (20-40 minutes)at around 4-5am ( just before sunrise ) .. and then 10-20 minutes sometime around 1-2pm when it is hottest.

But definitely never water after 5pm.

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If you water at night, the water has a greater chance of penetrating more deeply into the soil without being lost due to evaporation and transpiration from plant leaves. The downside is that you are more likely to have water sitting on leaves, and other plant structures that might promote fungal growth. So be sure to water the ground and not the plant.

If you water in the morning, you have to use comparatively more water due to faster losses due to evaporation. But you can water the leaves and wash off dirt on the leaves that might be interfering with photosynthesis. The sun will dry the water off lessening the chances of fungal attack.

Nature doesn't care. Rain falls day or night, and plants adapted to the local conditions do well.

You might see sometimes written that you shouldn't water mid day as water sitting on the leaves act as lenses and burn the leaves. That's a myth.

So, if there's a water shortage, water at night. Otherwise water in the morning. But water appropriately for the ground conditions, and weather.

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Watering in the morning gives your plants more free water to use during the day, watering at night can promote fungus growth due to excessive sitting moisture. Think like towels and mildew. Drop and wet heap of towels on the ground at night and by the morning they'll start to stink, but hang them to dry in the sun and they'll smell like the countryside.

As far as the plants go though it doesn't matter too much if once in a while you have to water them at night.

  • Also, if conditions are such that it's not as windy in the morning, there's less evaporation. This enables your plants to retain all that nice fresh water. – Sue Jun 17 '16 at 19:11
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The "don't water midday" rule comes from folks saying that water droplets focus the sun's rays and burn the foliage. My counter to this is purely anecdotal, but I've never seen leaf burn on anything and I have seen the positive effects of cooling off plants just after the midday sun bake is over.

Watering in the morning will allow a lot more water to get to roots, because it won't be evaporating at the staggering rate it does later in the day.

Target the entire plant. If it's fragile, just mist the foliage every now and then to clean it, and water the base deeply. If it's hardy then give it the full shower. Most plants respond well to being battered around by water (ime, Barberry, despite being a hardy plant, doesn't do well with recovering its shape, but almost everything else that appears hardy does, especially vines).

Only rule of thumb I see broken on a routine basis is: don't over-water. It's harder to over-water outdoor plants, but it's far from impossible.

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