Question: 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.
Learn how to improve your soil and thus improve the health of your plants—read Teaming With Microbes (Revised Edition) by Jeff Lowenfels and Wayne Lewis.
For under $10 you can get Healthy Soil for Sustainable Gardens, part of the Brooklyn Botanic Gardens' All-Region Garden Guides series.
Learn all about composting and choose the method that best fits your lifestyle with Composting Inside & Out.