Does hot or cold water affect plant growth?
While ice cold water can shock the roots of a plant, hot water
can actually be worse.
Boiling or near boiling water can damage the fine roots of
plants, inhibiting water take-up by the roots.
Room temperature water is best, especially for watering house
If you have a dried-out peat based potting soil, warm (not hot)
water can help the peat to re-hydrate.
Hot water on foliage can also be detrimental - many leaves are
sensitive to direct extreme temperatures, which is one reason that
boiling water can be an effective organic herbicide to treat small
weedy tasks, like grass growing in sidewalk cracks.