Bagged Planting Mix

soilQuestion: I was at the nursery this weekend and the fellow there suggested I buy a few bags of their “planting mix” to go with the shrubs I was buying. Is this really helpful or was he just trying to make an extra sale?
Answer: Planting mixes recommended for planting trees and shrubs generally contain an extended-release fertilizer and materials that improve drainage. They are designed to aid rooting and to ensure that water penetrates the root zone. The need for a special planting mix largely depends on your soil. If you have good, loamy soil, you can just mix some plain old compost in with it when you backfill your planting hole. If your soil is poor or heavy (i.e., clay), a mix is a good idea. Also, if you’re replacing shrubs—digging up old ones to put your new ones in—a mix will replenish the nutrients in the soil, which may be lacking, particularly if the previous shrubs were not fed or top-dressed regularly with compost or manure.

All that said, regardless of your soil—or if you’re at all in doubt of its quality—it’s not a bad idea to just spring for the mix. Compare the cost of a shrub to the cost of a bag of mix. A 32-quart bag of planting mix will cost you somewhere between $5 and $8 dollars; a decent-size shrub will likely start at 5 or 6 times that. So you’re not adding much to your bill—and it’s less than having to replace those shrubs if they fail. A 32-quart bag can cover two 2-gallon shrubs. You don’t have to plant the shrubs in it straight—you can blend it with the soil you dig from the hole.

I doubt the person helping you at the nursery was trying to sell you the bagged mix just for the sake of the sale. He was likely trying to be helpful and to better your chances for success with your new shrubs. In a roundabout way, this might mean more sales for him, since you’ll be likely to return to a nursery that sold you your garden’s top performers. More advice—dig your hole only as deep as the container is tall and two or three times as wide as the container, and keep the shrubs watered well (an inch a week) through the summer.
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3 thoughts on “Bagged Planting Mix

  1. Using a nice loose planting mix in the hole sounds like a good idea, but if the rest of the soil in the bed is heavy clay, you can create an area around the roots that all the water in the bed drains into because it can easily penetrate. Then you have a big wet sinkhole in which the roots can drown.

    The other thing you want to be cautious about when planting trees and shrubs is fertilizing. Fertilizers high in nitrogen (the “N” in the NPK analysis on the bag) promote the growth of leaves, when what you want in the first couple of years is to promote root growth. If the slow release fertilizer in the planting mix is something like 5-10-10, it’s probably OK, but I’d still prefer not to fertilize at all in the first year. Top dressing with finished compost is OK.

    • Thanks for alerting us to this. I like the Garden Professors blog, too. And here is a link to the original study –
      http://auf.isa-arbor.com/request.asp?JournalID=1&ArticleID=2499&Type=2

      It seems to me the study focused more on the ability of roots to push past the soil interface (where the hole you dig meets the rest of the earth). They were disputing past studies that suggested filling a hole with soil that’s different than the site soil inhibits root development past the planting hole. They found this not to be true, which seems to support the argument that amending the soil doesn’t hurt. At the same time — working with 5– to 7-cm caliper green ash trees in clay soil — the researchers also concluded that soil amendments did not quicken the trees’ growth, so it’s not as crucial as one might think. They did note, within the paper, that the clay soil dug from the hole naturally mixed with topsoil as the hole was being dug and backfilled, which to me sounds like a form of amendment.

      The study does say that a larger planting hole promotes faster root regeneration, and a hole with sloped sides helps roots penetrate the soil beyond the diameter of the hole. It didn’t affect top-growth as they expected it would, but it did make a difference to the roots.

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