Chapter 6 What Do Trees Need?

For long-time plant growers this might seem like a silly question, but all too often we forget to step back and tell novices about these essential things.

6.1 Light

All plants need light. It is the source of energy for photosynthesis, which is the collection of chemical processes by which plants convert low-energy carbon dioxide molecules into higher energy sugars. These sugars in turn provide the energy and carbon skeletons for building lipids (oils, fats, and waxes.) By combining sugars with nitrogen in the form of ammonium or nitrate ions, plants make amino acids (the building blocks of proteins) and nucleotides (which go into DNA and similar molecules.) These four basic materials (sugars, lipids, amino acids, and nucleotides) plus minerals and water are all plants need to grow.

Light is not just important for photosynthesis. Plants regulate their growth, flowering, and dormancy by sensing the brightness of the light, the total time they are exposed to light, and the relative balance of colors in the light.

Light is explored in more detail here.

6.2 Soil

Soil is the substrate that anchors plants and trees in place, and provides water and essential nutrients and minerals. Soil is not inert though. It is a mix of organic and inorganic particles, each contributing different physical and chemical properties. Biological processes involving micro-organisms and macro-invertebrates like worms further alter its properties. Good healthy soil and soil-less mixes help trees grow and resist pathogens, while poor soil creates a constant stress on the tree.

Soil structure is explored in more detail here. Soil mixes are described in depth here.

6.3 Nutrients

Macronutrients are the nutrients that plants transport out of the soil and use in larger quantities. They include nitrogen, phosphorus, and potassium. Micronutrients are required in much smaller amounts. They assist the enzymes that produce sugars, help build chlorophyll, and drive thousands of other plant processes. Most micronutrient deficiencies cause dramatic changes in the color and shape of leaves.

Nutrients are explored in more detail here.

6.4 Water

It is easy to forget how important water is to trees, but think about it this way. A tree can survive for many weeks in nutrient-depleted soil or poor light, while lack of water will kill a tree in just a few days. Over-watering a tree in poorly draining soil can kill it in a couple weeks or less.

Water quality is just as important. Most trees usually tolerate well or standard city water just fine, provided it does not contain excessive dissolved salts or minerals. Hard or salty water can damage and stunt roots. It is not necessary to go to the extreme of collected rain water. In fact this practice can hurt trees in urban areas because rain water can be contaminated with air pollutants like sulfur dioxide, which turns into acid when dissolve in rain.

How irrigation water behaves in containers is explored in more detail here.

6.5 Space

Just as trees need sufficient light, they also need sufficient space in which to grow. Trees being grown in the ground to fatten their trunks need space for the root mass to expand without competition, and for branches to grow freely without being shaded by other trees. Trees in pots on the bench need sufficient space between them to allow in light, and for air circulation. Lacking proper air circulation, trees become susceptible to a number of fungal and bacterial disease. If trees are crowded too close together on the bench, branches from adjacent trees can even block each others’ extension growth by triggering a hormonal response called thigmotropism (basically, touch-induced growth inhibition.)

Growing space is explored in more detail here.