Answer: Browning leaves on new plants typically indicates either over or under watering. Make sure you are keeping the soil for these new plants evenly moist, meaning damp like a wrung out sponge. It should not be sopping wet or dried out. Using a layer of organic mulch about three inches thick over the root area will help keep the soil naturally moister longer.
To know if you need to water, dig into the soil with your finger. If it is still damp, do not water yet. When you do water, apply it slowly and thoroughly at the roots so it soaks down to the deepest roots. Avoid wetting the foliage when you water. After watering, wait a few hours and then dig down to see how far the water soaked in; sometimes it can be surprising. It is better to water deeply but less often and encourage deep roots -- than to water lightly every day.
Iron is a micronutrient and a certain amount is necessary for all plants. Problems with iron are more often related to an "acid loving" plant such as an azalea being low on iron absorption due to a the soil pH being too alkaline. In a quick search I have not been able to find specific information about iron toxicity due to an excess of iron for hostas or hydrangeas. (In certain other plants, the symptoms of iron toxicity usually include stunting rather than browning leaves.) If the only plants having trouble are these new plants, I would tend to think the problem is related to moisture levels rather than the iron content of the water. If you have gardening neighbors with similar water you might want to check if any of them have had similar issues as well.
Another possibility for browning could be overfertilizing in general which can "burn" plants; other possible causes could include accidental herbicide or chemical exposure, spray toxicity, or excessive direct hot afternoon sun. (The hydrangea would do best in morning-only sun or in bright dappled light all day, the hosta is a shade lover.)
However, if you think you have been watering correctly and the plants are not located in too sunny a spot, then you may want to consult with your local county extension about the possibility of the water quality affecting your plants. They should also be able to tell you more specifically what is happening to your plants if it is due to some other reason, based on an overall photo of the plant and close-ups of the leaves and/or samples of the affected foliage enclosed in a clear plastic bag and kept cool so it stays fresh.
I'm sorry I can't be more specific for you but I hope this helps you trouble shoot.
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