I planted 55 apple trees in 2009 (bare root, heavily mulched) in relatively sandy slightly sloped soil.

also planted about 50 additional trees each of the succeeding years.

in each case the trees were watered weekly in their 1st year and periodically in the 2nd year if it was dry. most were on semi-d stock. If my lawn was dry I gave the trees additional water.

growth was disappointing the 1st 2 years. I started adding nitrogen in 2011.

last year (2011) was the wettest in many years. The trees planted in spring of 2011 did very significantly better their 1st 2 years than trees planted in previous years. (those trees did have additional benefit of add'l nitrogen)

what I think I've learned is that at least with my soil and sloping fields a lot more watering is needed for optimum growth. I am considering irrigating for the 1st few years even though it's not the norm for my area.

comments and suggestions are welcome.

Next year will be year 5 for my 1st planting (McIntosh, SnowSweet, Paul Red, Honeycrisp, & Zestar on semi-d) so I'm hopeful that even with the slow start I'll get a lot of apples next year. (So far none of the trees have yielded any fruit although this year was marred by a late frost.)

Irrigation will certainly boost growth the first few years. So that's probably a plus unless you end up pruning much of that off. You'll need to experiment some on how much water is needed after fruiting begins. Irrigation will likely increase yield and maybe even fruit eating quality. A lot depends on your soil and weather. Around here with our heat and normal droughts I need to water apples and pears quit a lot outdoors to maximize fruit quality.