The one thing most likely to cause an issue — like death — in your garden, is water: too much of it or too little. If you have an irrigation system, spring is a good time to think about what wasn’t working last year. Or what will need a tweak after a bed renovation.
Was there a soggy area in your lawn or garden last year? Perhaps that area is getting water from more sprays or emitters than it should. Get rid of the unneeded extra. Is there a dry area? Add a spray or emitter to correct that — or use the area for more forgiving plantings.
Remember that your trees may be thirstier than your lawn (or not as thirsty in the case of some native trees) — and they are much more expensive to replace. You may have to water evergreens even in the winter — under the eaves of your home, under the canopy of other trees… depending on your plantings, the amount of rainfall, etc.
Most of the water and air absorbing roots are in the top foot of soil at and beyond the drip line (the edge of the canopy) of the tree. So when you water a tree, you want to deliver water to the top FOOT of soil, at and beyond the dripline of the tree. NOT the trunk of the tree.
Is there a “formula” for watering deeply? It’s not the same as watering lawn. I can’t give you a formula on how to know when you have watered deeply enough. There are too many variables. And this is too important to fool around with.
– Give your drip line and beyond a soak, measuring the time watered.
– Cut into the soil — or just dig — in an easy-to-dig, inconspicuous place. (I like to pick a place where I don’t cut an irrigation line.) Do what you need to do to get a look at how far water has penetrated into the soil. Look and/or feel. Knowing how long you just watered, use that to adjust your irrigation plan for the tree.
E.g. A slow 30 minutes soak resulted in 4 inches of moisture penetration? Try watering for 60 more minutes and see if you get a foot of penetration in total. (Sorry but check a different test hole.)
Deep, infrequent watering is better than frequent, shallow watering. We all KNOW that, but we don’t necessarily DO that. I love to give things a “sprinkle” with my hose. But the way to make our water use count — especially for our trees — is to water deeply and infrequently (and slowly enough that there is no runoff).
**Don’t assume that soil is not dry just because you’ve laid down gallons of water or it has rained for a day. Be aware that when soil is very dry, it can become hydrophobic — a state where it repels water. This situation can easily arise on compacted soils, layered soils (topdress/mulch not intermingled with soil), under the eaves of your house, or under the canopy of evergreen trees. (Fine for the evergreen but not for anything else growing under the canopy.)
An example I saw compared healthy soil to newspaper. Newspaper absorbs water readily. Hydrophobic soil was compared to WAXED paper — water just stays on the surface and runs off. (http://www.soil.ncsu.edu/publications/Soilfacts/AG-439-25/)
The best way to see what is happening with water is to get into the soil and look. Even the greens at Crown Isle got cut into and checked if there were sickly patches. It let us really see what was happening when we watered the greens.