Question
Hello Sir, I have a wonderful evergreen growing in my backyard and I'm interested in knowing what time of the year it grows most.  I moved into this house 4 years ago, and the tree got 5 to 8 feet taller but I haven't started watering it until this year, I'm curious, do trees only grow in the spring?  I know it must depend somehow on what part of the country the tree is in, I live in Chicago, if I water the tree most in the middle of July to August am I wasting my time? Should I be watering the tree from early spring to mid summer instead? Do trees grow only in the spring? or both spring and all summer?

Also, will using all purpose Miracle Gro help the tree grow any taller?

Thank You.

Answer
To set the stage for our discussion of tree growth we must review some terminology.
Trees have six organs. These are: leaves, stems and roots (known as vegetative structures) and flowers, fruits, and seeds (known as reproductive structures). We will discuss vegetative growth--that is the growth of leaves, stems and roots. Tree growth is the increase in size and numbers of leaves, stems, and roots.

Growth occurs in meristems. A meristem is a tissue that contains cells which have the capacity to divide to make new cells. In general, during growth cells divide, cells elongate, and cells differentiate into structures such as roots and shoots.

Tree growth above ground includes shoot elongation (height growth), leaf growth, and diameter growth.
Shoots elongate or grow in height at the tips of the branches.  What happens is--cells at the apical meristem divide, elongate and differentiate. The steps that we can see are: (1) The bud at the tip of the branch opens, (2) Leaves emerge and enlarge, and (3) The area between the leaves expands (i.e., the stem grows). This is what you generally see in the spring.

Diameter growth
Between the wood and bark is a thin layer of dividing cells called the vascular cambium. This vascular cambium is a meristem which is only a few cells thick. This meristem divides, creating wood on the inside and bark on the outside.  Layer by layer these new cells increase the diameter of the trunk and branches. The wood cells are called "xylem" which means wood in Greek; they carry water and minerals up from the roots. The bark cells are called "phloem", meaning bark in Greek. These cells carry sugars and other materials produced by the plant.


Annual rings
New layers of wood are added each year between the bark and the previous year's wood. These are called growth or annual rings and may be used to age a tree. Cells that are produced in the spring are larger with thinner cell walls. These are the light-colored rings, and the wood is called "early" or "spring" wood. Cells produced in the summer are smaller, and this "late" or "summer" wood has a higher density and darker color.

Roots can grow in length (root elongation) and diameter, and new lateral roots can form. At the tip of a root is the root cap.  This cap protects the root and it must be constantly replaced as it pushes through the soil. Behind the root cap is a meristem which produces new cells for the root cap and for root elongation. These new cells elongate, divide and differentiate into root parts. The root grows in length and pushes through the soil. The majority of the root growth is in the summer and fall months.
So a tree grows all year long--height growth in the spring and wood growth in the spring and summer and root growth in the fall. A tree should receive 1 inch of water each week. so when it does not rain yes it may need water during not only the spring but the hot summer months.

If fertilizer is required, it can be applied any time from late September through early April. Often, best results are obtained when fertilization is done after the first hard freeze in October, but before the soil freezes in December. During this period, the soil temperature is still warm enough for roots to take up minerals, which are then stored for growth the following spring. Fertilizer applied during winter may be lost to leaching or runoff. The second-best time to fertilize is in March or April, just before new growth begins. Fertilizing after mid-July may stimulate late growth that can be damaged by an early freeze.

As a result, trees usually respond best to fertilizer with a 2-1-1 or a 3-1-1 ratio (nitrogen-phosphorus-potassium). Commonly available fertilizers with a 2-1-1 or similar ratio are 18-6-12, 12-6-6, 10-6-4, 10-8-6 and 10-8-4. Turf fertilizers usually work well for trees because they contain the desired ratio of nitrogen, phosphorus and potassium, and some of the nitrogen is in a form that is slowly released to the roots. However, never fertilize trees with fertilizer materials containing broadleaf weed killers such as 2,4-D or dicamba.

Use about 1 lb per inch of tree diameter scattered around under the foliage area of the tree and watered in good.