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Amendments at planting and backfilling

Loosen and break up large clods of soil before backfilling. Clods of soil used as backfill create air pockets around the ball and could hinder root growth and establishment.

backfillingAfter filling the bottom half of the space around the root ball with backfill soil and tamping lightly with your foot, push a hose with running water in and out of the backfill soil all around the hole. This will settle the soil around the root ball and eliminate air pockets. Washing the backfill like this may not be needed if the planting hole is at least twice the width of the root ball because a shovel can be sliced in the backfill to eliminate air pockets. Add soil so it is even with the top of the landscape soil. Do not cover the root ball with soil.

In all but exceptional circumstances where the soil is very poor, extensive research on trees clearly shows that there is no need to incorporate any amendments, fertilizers, living organisms, spores, dusts, powders, gels, humic acids, organic products, etc. into the backfill soil (Gilman 2001; Henderson and Hensley 1992; Ingram et al. 1981; Paine et al. 1992; Schulte and Whitcomb 1975; Smalley and Wood 1995). Water is the best amendment. Simply use the loosened soil that came out of the planting hole. The exception to this rule is where existing soil is so terrible or contaminated, such as in a parking lot island or in a small cutout in a sidewalk, that all soil over a large area is replaced with good-quality soil.

In addition, there have been huge growth responses to applications of spores of mycorrhizae-forming fungi in strip mines soils because many are essentially sterile. This is a good practice and is often recommended. However, there are few, if any, reports of a response in urban and suburban landscapes (more on mycorrhizae). A new development has occurred in 2010 which showed that collecting soil from around a specific tree species in an urban area and growing the associated mycorrhizae in a lab and then using that around newly planted trees on the same species can enhamce growth (Ferrini et al.). More research is needed to evaluate the efficacy and usefulness of this strategy, but is holds some promise.

Research on small seedlings showed that addition of the growth regulator paclobutrazol at the appropriate rate to the root ball at transplanting could increase the root:shoot ratio and this may reduce water stress (Watson, 2001). More work is needed on this before recommending it for landscape sized trees.

Slow release (or controlled release) fertilizer can be applied on top of the root ball and backfill soil or on top of the mulch at planting. There is no need to mix it with the backfill soil or place it at the bottom of the planting hole since most roots end up close to the soil surface in urban and suburban landscapes.

Under most circumstances, mulch will not steal the fertilizer from the tree (Gilman et al. 1990). Adding slow release fertilizer at planting has not been associated with either improved survival nor increased growth after planting in fairly good soil. There is little fertilizer research in poor soils, but more than a few horticulturists think plants may benefit from application at or soon after planting. It will not hurt the plant provided it is applied according to the directions on the product.

On the other hand, adding soluble fertilizer to a newly installed plant could burn roots. This will injure the plant and could kill it. In short, fertilizer is not usually needed at planting.

See: More on fertilization.