Am I overwatering or underwatering? (Or neither?)

This is really a pretty basic question. But my wife and I are a little new at this, having bought our first house a year ago. We're doing our best to renovate our yard, which was pretty neglected by the previous owners.
In our front yard, we have an ancient-looking but previously vigorous rhododendron, and nearby (about 6-8 feet away) I planted a fairly young (~5 feet tall) emerald green arborvitae last October. Both looked nice and green last year.
This spring, we've planted various other small flowers and grasses in the same area, following the instructions provided for spacing and watering. I've also given these flowers some all-purpose Miracle-Gro (again, diluted according to the instructions and given no more than once every two weeks) and I've thrown some "Holly Care" fertilizer around the tree and rhododendron, which are supposed to like that particular fertilizer (it's for acid-loving evergreens.) As the temperature has warmed up, though, both the tree and the rhododendron have started turning brown and losing some leaves. The rhododendron actually started growing a new layer of leaves on top, but all of these have now turned brown from the outer edge, with the rest of the leaf being a pale green. The older leaves seem to be ok so far, it's the new leaves that are looking pretty sad. The arborvitae is browning from the bottom, but I swear the rest of it also looks a little thin compared to when I got it.
The other plants in that area are too new for me to really judge how well they're doing; some do seem to be doing better than others, though.
We've had some weird weather in the past month or two here in the Northeast; it was bitterly cold for a while, then we had some torrential rains (like 4 inches in one day, for two days). Because it rained so hard for a while, I didn't water at all for about 10 days. That's when I started noticing the browning, and the soil was cracking from being so dry on top. I then watered well two days in a row last weekend, but I don't know if that was right or wrong. The soil itself is bone dry on top and never seems to stay wet, but underneath it feels moist (not wet). Keep in mind the rhododendron has been in that spot for years, and done well, so I don't think the soil itself is the problem.
My problem is I don't really know how much is a "normal" amount of water. My wife and I have actually been having some mini-arguments about it; she thinks that unless there's a drought, you don't really water at all because all of these things grew in the wild at some point. I think she's wrong, and in fact all the plants she tries to grow herself end up dead. (I of course point this out every time we talk about this.) I usually water trees once a week, and other plants a couple times a week.
So does it sound like we're overdoing it or underdoing it? How often should we be watering? (And maybe more important - who is right, me or my wife?)
Thanks...
Jeff
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To water, just water enough to moisten the non-woody absorbing roots. The non-woody absorbing roots absorb water with elements dissolved in it. They are mostly in the upper four inches of the soil. During drought the biggest problem is over watering. A very good question. I would be careful about using alot of nitrogen fertilizer about your trees. Large amounts of the element nitrogen can predispose your tree to sucking insects. The Christmas tree industry suffers from that predisposition often because the use large amounts of the element nitrogen to make the trees grow bigger and faster. That places the trees in a predisposition for sucking insects. Then they spray for sucking insect. Over fertilizing with the element nitrogen is very common. For more on predisposition see here. http://www.treedictionary.com/DICT2003/shigo/ARM.html
Sincerely, John A. Keslick, Jr. Arborist http://home.ccil.org/~treeman and www.treedictionary.com Beware of so-called tree experts who do not understand tree biology. Storms, fires, floods, earthquakes, and volcanic eruptions keep reminding us that we are not the boss.

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[snip]

How often should we be watering? (And maybe more important - who is right, me

1. There are a lot of variables that you haven't addressed, such as the type of soil you have, how long you're watering (ie, how much water are you putting out at one time), etc.
From the tone of your post I infer that you don't have mulch around the rhododendron. This plant should be well mulched -- 3-4 inches or more -- just make sure that the mulch is doesn't actually touch the trunk of the plant. That will keep the plant cool and moist without being soggy. It also sounds like you're using too much fertilizer and possibly too much water -- either of which can cause the leaf problems you've mentioned. Suggest one application of a good slow release azalea fertilizer before the plant goes into bloom and a second application after flowering has completely finished. That should do it for the year. Miracle-gro every two weeks is way over-kill, especially for an old established plant.
If the plant is mulched and you have sandy loam, water about twice a week if there is no rain, for 30-40 minutes with a good garden sprinkler or installed irrigation system -- not just a hose end into the garden bed. That should put out about 1/2 - 3/4 " of water, which is enough. You can measure how much water you're dispensing by putting out a tuna fish tin, cat food tin, or similar vessel and measuring how much water you collect in half an hour, then adjust from there. If you have heavy clay soil this schedule is too much water.
Your plants will probably improve with less attention -- just make sure they're weeded, mulched, and on a modest irrigation schedule, plus limited fertilization. Your local extension service probably has information on fertilizer and water schedules for your plants and your area.
Your wife is on the right track for well established trees and shrubs, especially if you have native plants. For annuals and newly established perrenials you will need to make sure they don't dry out -- but too much water will also rot the roots and kill the plants. Reminding your wife that everything she plants dies is not a winning tactic and will guarantee that you always work in the garden alone --
As for your second question, regardless of the facts in the case, you need to repeat about 10 times a day -- my wife is always right, my wife is always right, my wife . . .
Regards --
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Type of soil I actually am not sure about, which is why I didn't mention it, but also why I said the rhododendron has done well in this location for years. I don't think the ph of the soil is really a problem, although not knowing what type of soil we have is definitely one reason I'm confused about watering. It seems like we have several different kinds of soil in different areas of the house. The soil where the rhododendron is seems sandy, but I'm hardly an expert.

No - and I had planned to do this. I don't have much mulch in this area because it's right up against the house, and everybody advises against putting mulch next to your house in my area because of the risk of termites. But I can put a little bit down, I guess, and just keep the radius fairly small.
But the plant did well without mulch before. It's not like I removed mulch and now it's dying.

The Miracle Gro is being used on the other plants, which are new and growing, but I mentioned it because I thought it might be dribbling over a bit. (The MG package says once every two weeks for most plants.) I've only actually fertilized the rhododendron with the Holly Care, and I used less than recommended.

See, from this, it sounds like I'm not watering enough. I water this plant (and the tree) usually once a week, and less if it rains, and I don't water them for 30-40 minutes, that's for sure. I water them until the ground looks like it can't hold any more water, i.e. puddles start to form and don't immediately disappear. That's usually within 5-10 minutes. Sometimes I just go out there with a gallon can of water and empty it.
Should the soil be getting completely dry between waterings or should it be kept moist? And how deep are the roots of a rhododendron?
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[snip]

There's too much left out to even suggest a comprehensive answer. The reference to 30-40 minutes is for a sprinkler that wil add 1/2" - 3/4", but it sounds like you're using a hose end or jug to deliver a lot of water in a short time. How much sinks in? How much runs off? (ie, how much is actually available to the plant?) Is your soil heavy clay that may be waterlogged, or hardpan that allows all water to run off rather than be absorbed by the plant? Your rhodo needs both regular, deep watering, plus good drainage that keeps it from becoming waterlogged.
Here are a couple of references I found --
--Fraser South Rhododendron Society - Rhododendron Basics - What Rododendrons Require -- http://www.flounder.ca/FraserSouth/basics/requirements.asp
(" . . . [It] is important to understand their basic requirements.
First: Rhododendrons must have a constant supply of moisture. You may occasionally see a rhododendron that will survive without being watered, but it does so only under protest.
Second: Rhododendrons must never sit in stagnant water. Roots submerged in poorly oxygenated water will likely die, though a plant may survive through better drained surface roots. Hot, wet conditions are more dangerous than cool, wet conditions. That is why a rhododendron will survive in a wet spot in the Northwest during heavy winter rains but would not survive in a wet spot in the Southeast's heavy summer rains.
Third: Rhododendrons must be grown in an acid medium (pH 5-6) that is coarse enough for the roots to have access to needed oxygen.
Understand and provide these three conditions and you will succeed wherever you live.")
-- Clemson Extension Service: http://hgic.clemson.edu/factsheets/hgic1073.htm
("Rhododendrons are shallow-rooted plants and require irrigation during dry periods. This is especially true of those planted in the preceding spring. Rhododendrons planted in warm weather in sandy soils may require watering of the root system twice a week during the first year. Newly planted rhododendrons require regular watering during dry spells for the first several years. Pull back a small area of mulch beneath the canopy of the plant and check the soil moisture level. When the soil feels dry, wet it to a depth of 8 to 12 inches. Use soaker hoses or drip irrigation to slowly water the base of the plant.
Be mindful that rhododendrons in waterlogged soils will decline and become susceptible to root rot diseases. It is important to reach a balance between regular, deep watering and good drainage to promote a healthy plant."
-- Somerset County Chesapeake Watch Planting Requirements and Plans: http://www.somersetbaywatch.org/PlantingReqs.html
These should help you understand how to treat your rhododendron -- best of all, consult your extension service Master Gardener program. Regards --
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