Roots: anchorage, absorption, and storage

Picture a nursery worker easing the nursery juniper, this course's recurring instructional example, out of its black plastic pot. The soil does not tumble out. It keeps the shape of the pot, because roots have grown through all of it. Near the trunk, a few roots are as thick as a pencil, brown and woody. Everywhere else, the root ball is wrapped in a web of thin, pale roots, some ending in plump white tips where they pressed against the pot wall.

Above the rim, you can see the tree and judge how it is doing. Below it, out of sight, the roots work all the time. What exactly are they doing, and what do they need from you?

Four jobs underground

All of a tree's roots together make up its root system, and it does four main jobs.

• Holding the tree in place. This is anchorage: roots spread through the soil and grip it, keeping the tree upright against wind and its own weight.
• Absorbing. Roots take up water and the mineral nutrients dissolved in it. Nearly all the water a tree uses enters through its roots.
• Storing. Roots store carbohydrates, mostly as starch, and some mineral nutrients. The tree draws on them to grow new roots and shoots, especially early in the growing season and after losing roots or leaves (Lesson 9).
• Transporting. Roots carry water and minerals toward the trunk and receive sugars from the leaves to fuel their own growth. Water and sugars travel in separate tissues (Lesson 3).

Roots also make some hormones, chemical signals that help coordinate growth between roots and shoots.

In a bonsai, all of this happens inside a small container filled with substrate, the growing medium in the pot.

Structural roots and fine roots

Structural roots are the thick, woody roots that spread from the base of the trunk. They live for many years and thicken each year, much as branches do. They do most of the anchoring, carry water and sugars between the fine roots and the trunk, and hold much of the tree's underground starch. Their thick, corky outer layers let water through only slowly, so they absorb relatively little. On a bonsai, the flare of roots at the soil surface is the top of this framework.

Fine roots are the thin, numerous, branching roots that make up most of the web. Growers sometimes call them feeder roots, though they bring in water and minerals, not food. Young fine roots, especially near their tips, do most of the tree's absorbing. Many are short-lived: the tree keeps growing new ones while older ones die, and that turnover is normal. A few fast-growing fine roots keep extending instead, and over the years they thicken into new structural roots.

Growing fine roots end in pale, often white, root tips. Behind the tips, roots turn tan or brown as their outer layers toughen; that does not mean they are dead, and they still take up some water, more slowly. Even so, how well a tree absorbs depends less on how much root it has than on how many young, living fine roots it has, and on the conditions around them.

Where absorption happens: the root tip

The part of a fine root that absorbs most actively is a short stretch just behind its tip, too small to study in detail by eye. In “Zoom into the root zone,” start with “Whole root system” to find the structural and fine roots you just met. Then choose “A fine root tip” and select its parts in order, from 1, “Root cap,” to 4, “Root hairs.” Finally, choose “Root hairs in the substrate” and notice where the water sits and where the air is.

As the zoom showed, a root grows in length only at its tip, which is organized in zones. From the very end backward:

• Root cap. The root cap is a thimble of cells over the end of the root. Its outer cells wear away against the particles and are replaced from within. It also senses gravity, which helps the root keep its direction: main roots tend to grow downward, while many side roots grow outward at a shallow angle.
• Meristem. Just behind the cap, small cells keep dividing, adding new cells to the root and the cap. A meristem is a growing point; shoot tips and buds have them too (Lesson 5).
• Elongation zone. The new cells stretch to many times their length, pushing the tip forward. Most growth in length happens here.
• Maturation zone. Cells take on their final jobs, including carrying water toward the trunk, and root hairs grow.

Where a tree grows them, root hairs greatly enlarge the root's surface and its contact with the water films between particles. Each is a single cell of the root's outer layer, grown into a tube far finer than a human hair that reaches into narrow gaps. They help most with mineral nutrients that move slowly through the substrate, such as phosphorus, and with keeping contact with water as the substrate dries. Some trees, including pines, grow few and rely largely on mycorrhizae, partnerships with soil fungi whose threads take over much of that work.

Root hairs are fragile and short-lived, often lasting days to a few weeks, and new ones form only as the tip grows on, so a root that stops growing soon stops renewing them. Lifting, combing out, or cutting roots strips many away and breaks tips, which is one reason a tree can wilt after repotting even when its substrate is wet (Lesson 8).

Why roots need air as well as water

Root cells are alive. Like every living cell in a tree, they release energy from sugars through respiration, which uses oxygen and gives off carbon dioxide, day and night (Lesson 7). A root gets nearly all of its oxygen from air spaces in the substrate around it.

Substrate is made of particles, and the gaps between them are its pore space. A thorough watering fills most of the pores. In a pot that drains freely, gravity then pulls water out of the largest pores and air is drawn in behind it, while thin films of water cling to the particles and fill the narrowest gaps. A healthy, drained substrate holds both, as you saw at the last zoom level: water to absorb, and connected air-filled pores that let oxygen in and carbon dioxide out. That supply and exchange of air is aeration.

“Water and air between the particles” shows this at the scale of single particles. Move the “Moisture” slider one step at a time down to “Very dry,” then up to “Saturated for days,” watching the root and the two meters. Then set it to “Wet after watering” (the Wet mark), press “Water and let it drain,” and watch which pores empty first. Try the same from “Saturated for days.”

Only “Moist and airy” reads Good on both meters, and it is where a freely draining pot settles after watering. “Drying” still works, though the supply is shrinking. The two ends harm roots for opposite reasons: at “Very dry,” the last films cling too tightly to supply much; at “Saturated for days,” little oxygen reaches the root.

“Wet after watering” looks almost as full as “Saturated for days,” yet its root condition reads “Normal for a short time.” The difference is time. Harm comes when the substrate stays saturated for days: in a pot that cannot drain, a pot standing in water, or a substrate with too few large pores (Course 04, Soil, Substrates, and the Root Environment). It comes sooner in hot weather, and some trees tolerate wet roots far better than others.

Draining brings the air back and surviving roots resume their work, but dead roots do not revive, and neither do root hairs lost in a dried-out pot. The tree must grow replacements, which takes time and sugars from its foliage and stored reserves (Lesson 9).

Roots in a small pot

A container changes these conditions in three ways.

• A small buffer. A pot holds little water and air in reserve, so it can dry out quickly, and a pot crowded with roots often dries faster still.
• Uneven moisture. The surface dries first, while a wetter zone tends to linger at the bottom of the pot after watering (Course 04 will explain why).
• Walls. A root tip that reaches the pot wall often turns and follows it. Over time, roots can circle the pot, something growers deal with when they repot (Course 11, Repotting and Root Development).

Roots also keep their own schedule. They grow most in a moist, airy, mild root zone, neither cold nor hot, and not always when the shoots do: in many temperate trees, roots keep growing into autumn after the shoots stop lengthening.

How to check below the surface

Gently push an unvarnished wooden chopstick or skewer deep into the root zone, partway between trunk and rim, and leave it a few minutes: wood that comes out darker, cool, and damp has found moisture. Lift the pot right after a thorough watering, and again when the probe comes out dry, to learn how heavy it feels wet and dry. Tilt the pot to look at and touch the substrate at the drainage holes, and feel with a fingertip a little below the surface near the rim.

Each check has blind spots: a probe reads one spot, a large pot is hard to judge by lifting, and the holes and surface show the wettest and driest layers, so use more than one. Do not go by the surface or its moss alone; moss can stay green over a root zone that has dried. This is a starting routine, not a schedule, and Course 03, Watering and Daily Observation, will go further.