Give them water, but sparingly. A 24" tree is about what I grow in a 2 gallon pot. They get about a quart of water spread over 90 minutes when they get watered. I don't water until the top two inches are bone dry. You will probably be fine if you mulch them for about 4' around with black plastic to keep the weeds from competing with them. Planting in the field like this, when you water them give them about 1-2 gallons each. When you plant them, dig a fairly deep hole. (Power post hole diggers are great...) Fill in to the bottom of the pine roots, place the tree, and finish filling. You want some loose porous soil under the tree. Now water it well. Much of the water will end up under the tree. Which is where you want the roots to go. Be carefull of light watering. This will encourage the roots to stay at the surface. They become dependent on you, and when you forget, they die. I'm a bit surprised that you have white pine natively on sandy soil. Most white pines I've seen have been on moist lowland soils that didn't have much sand. White pine may need shelter to get started. Your conservation office will know. In any case I'd expect white pine to need more water than either red or jack. Other pines to consider are lodgepole and scots. Both are more symetrical that jack. The lodgepole tends to be taller and thinner.

You want the top-most root to be just below the soil. white pines are beautiful trees, my favorite pine and are easy to transplant as pines go. Water until established. Just feel down there with your fingers and if the soil is dried out then time to water. White pine is cold hardy and will only grow outdoors so that is the only option that you have. Plant it in the ground, it will do fine!

That's great, zone 8 sounds pretty nice! Right now, everything I have is indoors, under lights and in a heated room. :-) Last frost date will be less than two months away. It's way to early to put anything that's actively growing outside around here. Highs only getting into the 50's with mid 30's popping up a day or two in a row. Things don't really grow much in that kind of weather even if no frost at night (it didn't freeze the last couple of nights) and last year when I put new seedlings outside during the day, they went dormant and it took a while to get them going again. So yes you get to know how to judge your particular climate, just thought I'd mention that so the op would consider their area. :-) Even though I'm zone 3, near z4 border, I've learned that z5 isn't all that far ahead of us, maybe a couple of weeks earlier that we are. The main difference is the depth of cold during a somewhat longer winter. Once it gets to this time of year it's not a big of a difference really. :-)

I accidentally broke the growing tip off a determinate seedling (Latah) and had room so I planted it (June 30) to see what it would do. A lateral (sucker) did take over and when I transplanted it I pointed that straight up, the plant has grown but is much shorter than the others, 2ft tall rather than 3-4ft (I planted 2 other Latahs at the same time, right next to it, for comparison). It got flowers and now fruit on the tip of that lateral so I thought it was all over, that I would only get fruit from that cluster, there weren't any other flower trusses, but other laterals have grown taller and I have flowers on those. I will report back on fruit set. It will be much less productive than the other Latahs but may not be as bad as I originally thought.