Root Plates

Tree roots may surprise you.

Tree roots (AustinTexas.gov) Public Domain

We used to think trees looked pretty much the same underground as they did aboveground. We once thought trees sent taproots deep into the earth to suck up water and nutrients and hold themselves upright. All of those things are true except for the first one. Now we know that most tree roots extend laterally far more than horizontally. Most tree root systems extend 4 to 7 times the diameter of the aboveground portion. The root system of a mature oak can extend up to 250 feet! We also know that they communicate and exchange nutrients with neighboring trees, but that's another discussion. Today, our garden topic is root plates.

What are root plates?

Root plates are the combined disk-shaped mass of roots and soil immediately surrounding the trunk of a tree. If you have ever seen a tree knocked over by a strong wind, you have probably seen a root plate. ​

Dried root plate of European spruce (H. Zell) CC BY-SA 3.0

These structural roots and soil form massive disks. As tree roots spread out just under the soil surface in search of water and nutrients (and possibly neighbors), they quickly taper as they grow away from the main stem. The root plate edge occurs where the roots shift from thick and stiff to somewhat thinner and more flexible.

Generally speaking, you can assume that for every inch of trunk diameter at chest height, these critical roots extend one foot in every direction. Smaller transport roots emerge from these structural roots to carry the food and water collected by fanning absorbing roots. These secondary roots help delay the symptoms of damage to the root plate. But they can’t carry the extra burden indefinitely.

Trees store approximately 2/3 of the energy they create in their root systems. If those roots are severed or damaged, those stored nutrients are lost. Eventually, there is a deficit. Symptoms of a damaged root plate may take three to seven years before symptoms appear. By then, it may be too late.

Nearly all tree roots are in the top 18 to 36 inches of soil. The combined mass of root material and soil is called a root plate. Because most tree roots are shallower than we thought, we now know that tree roots are more susceptible to our actions than we thought. But there are benefits.

The benefits of wide, shallow rooting

Trees with shallow roots spreading out in all directions have better access to the nutrients in decomposing leaf litter and other plant debris. They are also close enough to the surface for critical gas exchanges. Trees need nutrients, oxygen, and water to grow. Deep soil does not contain enough water or oxygen for trees to thrive, so they look elsewhere.

The downside of root plates

Shallow-rooted trees are more likely to fall over. A strong gust of wind can tear the root plate free from smaller roots. The entire disk of roots and soil can slip, spin, and upend a tree. We’re talking about massive force when trees break loose. And root plates are equally impressive. For example, a root plate that measures 11-feet across and 2.5 feet deep would have an average volume of more than 270 cubic feet and weigh more than 14 tons. You don't want any of that falling near your home or car.

Root plate of windblown pine tree (Ballogie) CC BY-SA 2.0

How can we help our trees?

So, what can we do to keep our trees from falling over?

• Mulch 2-4 inches deep around trees, but not touching the trunk.
• Avoid rototilling or digging near trees.
• Top-dress around trees regularly with aged manure or compost to maintain healthy soil and provide food for your trees.
• Minimize heavy traffic directly under trees.
• Install trees where their roots won’t damage sidewalks, driveways, or building foundations and need to be cut.

Did you know that corn, sunflowers, and wheat have root plates? I didn't either.