When should I water my plants?

There seems to be much debate over when is the best time to water the garden. Some say morning is best …. others say evening is their choice. So what’s right? Does it really matter anyway?

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Well I will set the record straight now … I never water at night, morning is always my choice. It’s all to do with “turgor” and “transpiration”. What the heck???

Yep you read it correctly … turgor is the pressure exerted by water inside the cell of the plant. A plant is said to be “turgid” or standing up without wilting because of the pressure that this water places on the cell wall. The water is drawn into the cell by the plant when it stores sugars and soluble nutrients during photosynthesis. Nutrients are absorbed through the plant roots by osmosis and then flow through the vascular system into the stems and leaves. Because a plant doesn’t have a heart to pump the nutrients through like us, it moves the nutrients around by capillary action. It’s almost like a push-me-pull-you action.

When we sweat, perspiration is evaporated from the pores on our skin. Similarly, plants transpire and lose water vapour from their leaves and stems through small pores called stomata. The amount of water in the soil and temperature of the soil will alter the plant’s ability to open the stomata more and increase the rate of transpiration. Transpiration will pull water through the plant, while osmosis pushes it into the roots.

Water in the plant, carbon dioxide from the air and light from the sun are used for photosynthesis, ie. to produce sugars or energy for the plant to grow. This only happens during daylight hours so water is only drawn through the plant in the day, not at night. If you water the garden early in the morning, the plant will actively draw moisture into it’s cells and will quickly become turgid. A well hydrated plant will suffer the extremes of a hot day much better than one that is not. Just like us, if we go out into a hot day without enough water, we soon end up with a headache and have a hard time recuperating from the heat. If we go out fully hydrated to start, we might feel a little “wilted” in the heat of the day but we quickly bounce back. Moral of the story: water the garden and yourself before leaving home!

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Now here’s another term for you – “guttation”. It sounds like a digestive problem but it’s not. Have you ever seen tiny water droplets forming on the tips of the leaf early in the morning? Well, this is known as guttation. Over night, the push-me-pull-you action stops because the stomata are closed, but if the soil is really moist then there is still pressure pushing water into the roots. The water accumulates in the plant and is forced out through tiny water glands, forming droplets. Guttation is a result of the water pressure pushing rather than transpiration pulling it through the plant.

At night, when we’re closing our eyes to sleep, a plant is also closing it’s stomata and has stopped photosynthesis. Now if you water the soil at night, the water will stay in the soil or on the plant and not be drawn up by the plant. This creates moist, humid conditions overnight … just the right sort of environment to breed fungal diseases on your plants during a hot balmy summer night.