You have a higher risk of fungal diseases if you don't give your plants a chance to dry off before nightfall, so morning really is best. Do it early and it'll have time to soak in before evaporation becomes an issue.

I agree that morning is best, but evening is second best. :)
And definitely water at as close to the roots as possible to prevent fungal growth.

I like to water in the morning, but do weeding, pruning, picking, and other tending in the afternoon. I feel my plants need a "pick me up" in order to make it through a hot summer day. But an evening "check in" enables me to get to know the neighbors and relax at the end of a long day.

I think it depends on what type of sun exposure your plants have. Personnaly, the ones that get direct sunlight all day I actually sometimes need to water twice per day (it was worse when I had those horrid plastic containers that dont retain moisture. definately invest in ceramic). If I water them in the evening at around 7pm there there is still another good two hours of daylight for them to soak it up before nightfall. But I usually try to water in the morning. I think this will be the type of thing you will learn trial and error. This is my third year with container plants. The first a lot of plant death occured...

I've been doing it in the morning with the idea that it simulates morning dew. I figure if it's going to get really hot, they'll want the extra water before the sun starts evaporating everything. That, and I'm usually sweating after a morning run so I want to turn the hose on anyway to douse myself.

Watering in the morning is best. But if your containers dry out quickly, you'll have to water them in the afternoon, also. Just try to keep the leaves dry or your plants could end up with powdery mildew and other fungal diseases.
The 6x8 Garden

So much of this depends on your location, the weather, and the flower type you're trying to water. But aside from that...

In Missouri, I water both in the early morning and in the early evening because of the intense summer heat. I water from the bottom only in the morning to avoid burning any stems, roots, and leaves on the plants. In the evenings, I split the water in half, watering 50% from above, and 50% from below. Containers dry out really fast in the heat, so you may have to adjust your schedule and add in a morning cocktail to get the job done. :D

In cooler climates, I think you could easily get away with my evening method of watering once a day.

Good luck!!

Morning is preferred to prevent mildew and fungus, as many have already commented. I often have to water mine twice a day given the blasting heat they get sitting on my south-facing front porch

In Georgia I set the drip irrigation for 6:00AM and 6:00PM watering daily. If I only water once a day, the leaves will droop! Your climate may not necessitate this, in which case, water in the morning.

The morning is my preferred time. On especially hot days, it is better to give the water BEFORE they get parched and begin wilting and have to spend valuable energy recovering. Though, if the soil is dry after a hot/sunny day, an occasional light watering before sunset won't hurt. I also try to take into account recent or coming storms.

Early morning is best-- that way the plants have a little time to soak it up before serious heat sets in.....

Yes, morning is best for the health of the plants.

But if your plants dry out and need a second drink later in the day, I think the benefit of some water outweighs the risk of disease.

We live in central Pennsylvania, and it frequently rains in the late afternoon during the summer months. To avoid water-logging, we water in the early evening so we can assess moisture levels more precisely.

Morning, not evening. The roots are growing during the day and need the moisture, the leaves do most of their growing at night. The only real reason not to water mid-day is because it is a waste, a larger percentage of the water is being evaporated immediately.

I water mine at any time of the day I have the time. My work schedule gets me out of bed at noon and home at midnight. So my "morning" is bit too hot and my "evening" is Fungus Time. I just try to get them water on my days off at a reasonable time, and just top them off at noon or midnight only if I have to.

I never thought plastic pots dry out, I thought it was just the terra cotta. But I'll try ceramic and see if that works better.

I'm in northern Cal, so I think that makes the plants more forgiving. In Texas, I had to get them in the early morning or early evening and hope they didnt' mold

I water in the morning because watering with not a lot of heat left in the day causes damp conditions which leads to fungus and mildew problems. When you get water on the leaves it ends up just sitting there over night, and we had fungus wipe out our entire pumpkin patch.

I live in a dry climate and it often gets in the 90's during the peak of summer. What I've learned is to water the plants in the mornings especially new plants until they get established then after they are established I often water in the evening which gives them a chance to take in the water with less evaporation. Still, when the forecast is calling for high temps for a number of days in a row I will occasionally water in the mornings or fill just the trays with water to provide some humidity. I do all this even with only northern exposure because the dry air can really deplete soil of moisture especially in containers.

After reading some of the other comments I'm glad I don't have to deal with or worry about fungus. I guess the bottom line is it depends on the climate where you live.

when i had a container garden on my 19th floor balcony in Ottawa, i had to water twice a day. my sister or i would water in the late afternoon as soon as we got home from work.

My south-facing balcony becomes really hot so twice a day is often necessary, but if it's not the warmest season then only in the morning.

Containers? At least twice a day on hot sunny days, only once if it's cooler or overcast. Preferably morning. In pots with drainage. Use moisture control crystals if possible, they retain some of the moisture without sogging up the soil, so it can slow-release as needed.

One of the best tricks I learned somewhere was putting an empty plastic bottle (mine once held sparkling water), stabbed many times with a steak knife to create a bunch of slits top to bottom, down the center of my strawberry jar before filling with soil or plants. The mouth of the bottle is flush with the soil on the top of the pot. I fill the bottle with water and it drains into the pot through the slits evenly to water the pockets at the bottom as well as at the top.Works great, I have red berries out there waiting to be eaten!

I'm just getting in on the container gardening bandwagon. I'm starting everything from seeds, so I have tiny starter pots in the window. They get direct sunlight from about 1pm until sundown, so I water in the morning before going to work and just give a few drops to keep the tops moist before I go to bed.

So far, so good. Day 4 and I already have sprouts!

My father always waters in the evening and we never had any problem with diseases. And our garden is really lovely. If you prefer evening why don't you try to water in the evening and watch your plants carefully and if you detect any problems like fungus you still can switch but you might as well never have those problems.

I usually water when I get home from work, early evening. I've never had problems with the plants, watering at that time...besides, I get up too early and have to head out to work in the morning.

I water my plants when i return back from my office...in the evening...

Although it presents more positives, morning watering is just not practical for me in my current lifestyle. I surely could, but I would either be dragging the hose out at 7am in my PJs or work clothes, not ideal- for now.

I do it in the evenings, right around dusk is about the time I finally have a chance to get out to my city patio and water the plants. I have a variety of leafy green plants, flowers, veggies and herbs (oh, and the ficus and ivy wall!).

I usually am socializing with the neighbors, maybe even enjoying a glass of wine when I do it. It's a nice way for me to wind down from the day and I have really come to look forward to doing it in the evenings. Some "me" time.

For me, the morning would be rushed and I would probably not enjoy it as much. Evening is what works best for my lifestyle. The plants haven't complained yet, and if they're lucky, I am clumsy enough to give them a drink of the good stuff in the process!