Arbor Day is Coming: Tree Planting 101

Posted by: Alison O'Connor, Larimer County Extension

My body hurts. Carol was right. I spent the day in the garden, doing my spring clean-up and towards the end of the day, I thought to myself, "Well, I might as well plant that linden."  You might remember my previous linden, the one that was a hot mess--sunscald, girdling roots, planted too deeply. Through an act of utmost generosity, Dr. Klett donated a linden to me to replace the one that met its fateful demise. I'm bound and determined to make sure my new linden has a healthy and long life. Just like all the trees you may plant this spring for Arbor Day.

So how do you plant a tree? Well, it really is more than digging a hole. If you're a Master Gardener, you've probably sat through my 3-hour lecture on tree planting, but here's an overview (with pictures!) to help you plant your tree correctly for long-term success.

Before you even THINK about planting, make sure to call 811 to have your utilities located. This is very, very important, especially since March Madness is occurring and cutting your neighbor's cable line will be bad news.

First of all, my tree had some structural defects, so I did some pruning to remove broken branches, those that were rubbing and picked a new central leader. In most cases, you should not have to do this. Your tree should come properly pruned from the nursery. With this tree, the need for pruning had been overlooked for a couple seasons.  I removed quite a bit of branches, but for me, it's easier to prune prior to planting to correct these problems than wait until they are a major issue.

I knew the site I wanted to plant my tree in (the same place as the old linden) and the first step was to determine how deep my hole needed to be and also how wide my hole needed to be. So I measured the height of the root ball and also the width of the root ball. The hole should be about three times as wide as the root ball, so just multiply by three. The depth of the hole needs to be 1-2" shallower than the root ball. Yes, the tree will sit 1-2" above the soil grade (trust me on this). The reason for this is because we want the structural roots to be right at grade (not too deep or too shallow).  Roots need oxygen to function, and in our clay soils, the deeper you go, the less soil oxygen is available (due to small pore space). So make sure your tree sits above grade.

You should also check to make sure that the structural roots (the big 'uns) are only 1-2" deep within the root ball. Use a thin screwdriver to check. You'll know you've hit a structural root because your screwdriver will meet a solid point of contact.  In my case, this linden was planted properly and the structural roots are 1.5" down in the root ball, so when I plant this tree, the roots will be at grade when I finish.

Now that you've determined your depth and size of the hole, start digging. And digging. And digging. The root ball on this tree was enormous and the hole was nearly 6 feet wide, tip-to-tip. The hole should be saucer-shaped and more shallow than deep (the hole width is key to rapid root regeneration). Make sure you check your hole depth from time to time, to ensure that you don't dig too deeply. Lay something flat across the hole and measure the depth. The soil at the bottom of the hole should be firm--stamp on it with your foot. If you leave the soil fluffy and loose, the tree can sink as gravity takes over and you water.  Again, the tree needs to sit on firmed/un-dug soil.

Remove the tree from its container. Wiggle off the container...or cut it off if it's being stubborn. If you're planting a balled-and-burlapped tree, leave the wrappings on the root ball until you get it into the hole--you don't want to risk the root ball shattering and potentially damaging the root system.

And what do we see after removing the container? Jeepers creepers, a mess of circling roots on the outer periphery of the root ball!

The old recommendation was to take something sharp and slice through the roots. Unfortunately, research proved that this was ineffective (check out Jeff Gillman's blog on the subject; also see Ed Gilman's work). The new recommendation is to shave off about 1 inch of the outer periphery of the root system to physically remove the circling roots. Let me get my saw...

So I shaved the root system (bottom too). I know, it seems...wrong. But truly, in the long run, it's going to be better for the tree. Be ruthless. The thing with circling roots is that those little roots will get bigger. They can turn into girdling roots, which can affect the tree's health. In fact, there's a thought that many tree failures are now a result of girdling roots, which act like a boa constrictor on the trunk, cutting off water and nutrient transport. Plus, circling roots (the existing ones) will not grow laterally into the site soil. After shaving, I had a pile of roots:

Now that I've shaved the root ball, the tree is ready to plant. Heave ho into the hole! Check it from several angles and make sure it's nice and straight. Also make sure the root ball is sitting 1-2" above grade.

Once you're satisfied, add a shallow ring of firmed soil at the base of the root ball. Doing this will eliminate the need to stake in most cases. Yes, it's true! The shallow ring of soil is going to keep the tree in the ground and prevent it from rocking back and forth.

Staking can actually damage the tree. There are really only three reasons to stake following planting: windy sites, near people activities or for a tree that cannot support its own weight. For the most part, a tree in a homeowner's yard should not need to be staked.

Then add your backfill. I always get the question about amending backfill soil, and really, science has proven that it's not beneficial long term. The rule of thumb with organic matter is to have 5% in your soil. Adding a "rich compost" to the backfill is essentially planting your tree in a container. Plus, too much organic matter can be detrimental. It can hold too much water and decomposes more rapidly. So I'm not an advocate of adding copious amounts of organic matter to the backfill, unless you have a really awful soil. It's more important to plant the tree correctly than to rely on organic matter to make things better. You'd be surprised at how successful roots are at growing into site soils.

When you're backfilling, do not put any soil on top of the root ball. If you do, there's a good chance that when you water, the water will skirt around the root ball. Water is lazy. It's going to take the path of least resistance, which means it doesn't want to go through soil texture interfaces, like the one with the root ball (dark, rich soil) and the native site soil. Keep the soil off the top of the root ball.

After adding the backfill, water gently to allow the soil to settle. Then add more backfill, water and repeat as necessary. You may have to do this several times to get the tree to final grade.

Finally, add organic mulch. It helps keep weeds down, stabilizes soil moisture and also adds organic matter as it breaks down. Just like the backfill, keep all mulch off the top of the root ball. The mulch should come up to the edges of the root ball, but not cover the top. Mulch on the top of the root ball will not decrease moisture requirements, but it can increase circling roots. When you're finished, you should clearly see the root ball, which makes it easy to water, since you can focus your efforts for the first season on just watering the root ball. After a season or two, start watering the planting hole and beyond. The rule of thumb is to apply 1-2 gallons of water per inch trunk diameter as needed (don't let things dry out), maybe 3-4 days/week, depending on the temperature and wind.

And you're done! Congratulations! The CSU Garden Notes also have a great publication on Tree Planting Steps. Don't worry....I will remove the tree wrap by mid-April.