Answer: Loosen the soil over a wide area, two to three times wider than the rootball and about as deep as the rootball. Leave the sides of the hole rough. Set the plant in the hole so it is at the same depth as it grew before. Refill the soil, then water slowly and deeply and thoroughly to eliminate air pockets.
Mulch with several inches of organic mulch in a flat layer over the entire disturbed area. This will help keep down weeds and help keep the soil moist and also feed the soil slowly as it breaks down over time. Keep the mulch in a flat layer about three inches deep and do not pile it against the trunk or stems of the plant.
Do not fertilize at planting. You could possibly fertilize next spring and/or apply a top dressing of good quality compost. If you fertilize, use an all purpose slow release granular fertilizer such as 10-10-10 per the label instructions.
The most important thing you can do for this fall (until the ground freezes) and all next summer is to water if needed. 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 water, water slowly and deeply to encourage deep rooting. It is better to water deeply but less often than to sprinkle lightly every day. After you water, wait a few hours and then dig down to see how far the water went; it can be surprising.
If these are in a windy spot, you may want to make a wind break for them this first winter since you are planting them in the fall and they will not be fully rooted before winter.
The spacing depends on what cultivar you are planting, some are much wider than others over time. If the mature width is listed as four feet, for example, plant them three to four feet apart. If they are too crowded they will not be as healthy and vigorous in the long run. Enjoy your arborvitaes!
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