We planted 17 emerald arborvitaes 3 weeks ago. We broke apart the rootball (at the suggestion of an employee at the retailer) and I've since learned that this is bad to do. We are watering regularly and have covered the area with mulch to retain moisture but our trees seem to be yellowing slightly. What can we do to encourage survival? I'm worried we may have shocked them too much. Any type of fertilizer we can use to encourage root growth/provide nutrients?
Poor planting is by far the number one problem with all trees for many years after planting. Surprisingly, it is better to correct bad planting by re-digging and replanting trees for up to 18 months after poor planting.
That said, be aware that healthy new growth for arborvitae is light green to yellow-green in color, and will darken considerably as the season progresses. So don't panic yet. See if the yellowing tips are new growth; if last year's growth is yellowing and begins to turn brown and dry, that is the indication that you have a problem.
The solution to poor planting is not to add fertilizer or other nutrients. Nothing will compensate for bad planting.
Current best practice for tree planting is very different from older advice, and unfortunately it is common to still find outdated advice printed on the grower's plant tag. What recent research has shown conclusively is that proper planting means digging a broad, shallow hole, gently spreading out the roots, and planting the tree high, with the highest roots right at the top of the soil. Then back-filling with unamended soil--no compost, no fertilizer, no manure, no amendments or additives of any kind. Then gently watering the tree to saturation, and placing about three inches of mulch (bark dust, tree chips, or equivalent--not compost) on top, from well beyond the drip line inward almost to the trunk. The old advice of digging a hole as deep or deeper than the pot or burlap ball, and somewhat wider than the pot or ball, then backfilling with soil that has been enriched with compost, fertilizer, or other amendments has been decisively shown to result in substantially reduced plant health, especially as this "deep hole" strategy often results in the softened earth settling beneath the new plant, sinking the area between top roots and trunk too deep into the ground. And the amended backfill actually discourages growth of new roots in a surprising way: the living plant continues to pull water out of the original rootball, but this water is not replaced by irrigation water, which can saturate the amended soil without being absorbed by the original rootball--so the plants original roots frequently die of drought even though the plant is actually receiving adequate irrigation (or rain) water.
Breaking apart the original rootball is frequently a bad idea, because the dirt broken away may contain a substantial percentage of the transplant's existing roots, which have already been seriously reduced by the process of field-digging the plants for transport and sale. On the other hand, it is often excellent practice to gently wash away a substantial portion of the original field soil, as this allows the roots to be spread out as widely as possible in the broad, shallow planting hole, and to encounter as much as possible of the original, unamended backfill.
Surprisingly, research has shown that it is better to dig up and properly replant most trees for up to a year or 18 months after they have been improperly planted than to leave them poorly planted. Longer than that and the situation becomes dicier, but trees that have been planted poorly --especially those planted too deeply-- often have a much reduced lifespan even if they do not die initially.
Additional comments from Al Shay Mag, Horticulture Instructor:
Well, unfortunately you could not have picked a more sensitive candidate for root demolition than Arborvitae. Evergreens in general do not have a fail-safe as do most deciduous trees with latent buds, allowing them to resprout. The planting date was much too late to ensure a high level of success for this plant, they need plenty of water to become established. February would have been a much better time. As stated in the original answer “it’s best not to disturb the root ball of an evergreen”. Contractors and homeowners have lost these trees simply by dropping them out of the back of a pickup. All of the recommendations listed make good sense. I would certainly complain to the retailer as this person obviously was not a trained horticulturist and should not have been giving advice. Wish I had better news, hope for the best.