Well, first there is the evaporation rate. With the sun shining and the heat of the day the evaporation rate is much higher. Another reason is that when the sun hits the water- if it were on the leaves of the plants, it would concentrate the sunlight and burn right through them like fire. You know what I mean? or do you mean something else?
I did not water this morning because I gave everyone a good long day before yesterday and hand watered yesterday. But when I checked the 24"box that two jalapeno's are growing in, the meter pinged dry so I dragged out the hose. It's 118° here now, in the shade of the patio, so the small area of the garden where this box is gets more sun likely is hotter.
What Torrie said about being careful not to spray the leaves. The other factor is the water temperature in the hose. When I first run it, the water coming out will scald my hand. I run until it's cool, then avoid getting drops on the leaves so they sun won't burn through - creates a mirror as you may remember from scouting days.
I think if you keep at least those facts in mind (maybe there's others and someone else will come along and enlighten us), I figure it's better to water at 2pm than to let them die from lack of hydration.
"....avoid getting drops on the leaves so they sun won't burn through...."
I have heard this (and read it in published accounts) before. It isn't true. Hold a magnifying glass lens in your hand and let it be hit by the sun. It won't burn because the focus is somewhere further away than a lens in direct contact with your skin. The holes you may see in leaves were put there by insects not water droplets.
Heck yes, if you spot an aw shit tree wilting better to hit is with water immediately and save some feeder roots than waiting until the next morning. As you said, after running off the hot water retained in the hose/water line soil you don't cook the poor tree's roots. I usually dump it in the yard where only a mature tree's roots (or grass resides).
You can water whenever you wish. Especially if your soil does a good job storing water. I used to claim water uptake required photosynthesis. It does not. It can occur at any time and is transported from roots throughout the plant through at least 2 separate pathways (hydraulic lift being the other).
Reasons to water early morning:
1. Cooler so less water lost to evaporation.
2. Less wind (not in my experience) in the morning so less evaporation and less carry over to leaves and trunk if sprinkler system utilized.
3. Water transport to leaves requires stomata (pores on the leaf underside) to be open with evaporation occurring at these opening (water drops falling from trees). When temp is over 85F stomata start to close off to conserve water thereby negatively effecting water uptake. (....and CO2 uptake and waste O2 gas exchange and thereby photosynthesis is shut down).
hmm...I'm skeptical on number 3. The lows are my house are around 90 but I see lots of new growth on certain plants outside. Some heat loving plants thrive (guava, sapote, oleander, pom's as examples) which would suggest that with a good amount of water (happening when I damn well feel like it) will allow a plant to thrive.
I guess based on the replies here, that watering in the morning is a myth? That's what I'm after.
My take on "don't water at night", if that's what your question really is, applies more to other areas of the country than us. What else is new, eh? What I have read is, evening/nighttime watering invites mold to develop. I *seriously* don't think we need be concerned about that here. I water in the early mornings because that is what is convenient for me. Sometimes I'm not home from the commute until 5 or 6 pm and need to get dinner started. I'm up at 4 or 5am, it's light by 5, I'm out there with the water wand and/or small sprinkler(s).
Hope this helps Brian. Powell, that's interesting on the no-leaf burn issue. I thought it made sense. Oh well.
ha! I love when people give the answer that it makes sense for other parts of the country. I love talking to my uncle in Virginia who is an avid gardner and we just speak another language when it comes to simple things like Tomatoes or lettuce or whatever. I have no idea how he grows anything. He makes no sense. Never heard of mold due to watering.
"I thought it made sense."
Me too until I had it explained as I explained it to you. But the salt residue left behind IS not good.
I think the filter I have on the hose helps control the salt.
You can. In fact, I did it yesterday during the peak of the heat to save a few newly planted trees that were starting to head down the slippery slope. It's more efficient to do it at night, as you'll have minimal losses due to evaporation. As a few people have said, you generally don't want to spray the leaves during the day with our salty water. It can cause leaf burning or clog stomata due to the high solute load .
Brian, my recommendation to folks is to change the watering timing depending on the season. Hard and fast rules can get us into trouble particularly when they come from the rest of the 4 season country gardening gurus.
As the others have mentioned there are a lot of 'signs' you need to pay attention to. Wilting during the day in our heat is not uncommon as the plants may 'droop' to retain moisture, but if a plant is obviously distressed a slow trickle at the base or drip line will help. Water does not move 'horizontally' through soil unless it is being flooded from the surface (like our irrigation systems), so if the plants are distressed you need to get the water to the roots.
If you have young plants you can do a wicking bottle of water to trickle in (fill and upend any large bottle of water - hold your fingers over it and 'screw' it into the ground, propping it up if you need to) - the water will slowly soak in. You can use it with containers as a vacation waterer.
In the summer water in the evening/twilight to allow maximum absorption and minimize evaporation.
In the winter water in the morning to minimize mold and mildew buildup.
It is always a good idea to avoid getting water on the plants, rain is different from our hose watering.