It doesn't matter. It's the force of the water that counts.

If the hose has been in the sun, run enough water to ensure you don't cook your plants.

lazy_gardens are you "99 44⁄100 percent " confident that the pressure to remove an aphid at 90℉ is the same as needed to remove an aphid at 50℉?

I would never spray any of my plants with water from a hose that had been laying in the sun without first running that hot water out of the hose first since hot water is used to kill unwanted plants.

I have used water straight from the cold water tap (about 55 degrees F) numerous times to knock unwanted critters off plants with no harm to the plants.

rhizo_1 (North AL) zone 7 Albert, you are being silly. Frequently. But usually not entirely.

I really do wonder if aphids on plum trees are easier to wash off when it is chilly than when it quite warm.

I found one site that said spray with 'cold water' but did not elaborate. It would be unreasonable to use water any colder than tap water, I should think. But what about the temperature of the aphid? The leaf? I am assuming they are what we called 'cold blooded' in grade school. ''Poikilotherm'' Is the technical term, perhaps.

I have a very sensitive infrared thermometer and "being silly" I have noticed that in the morning the tops of leaves hit by sunlight were measurably warmer than the bottoms. The aphids were all on on the bottom side of the leaves.