How Do Tree Growth Rings Form?
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How do tree growth rings form? Growth rings form in a thin layer of dividing cells called the cambium, which adds new wood each year. In temperate regions the cambium becomes active in spring and slows in late summer, so each ring pairs a lighter band with a darker one. This explainer answers the question directly, explains the mechanism, and notes where the sources stay general.
Quick answer
- Growth rings are formed on an annual basis by trees growing in temperate regions.
- The cambium is the region of a tree where cell division adds new wood.
- Cell division starts in spring, proceeds rapidly, slows in late summer, and stops in the fall.
- One ring pairs a lighter springwood band with a darker summerwood band.
Where the ring starts: the cambium
The cambium is a thin layer of actively dividing cells found directly behind the inner bark of the tree, or phloem, according to the Illinois Extension answer. The Illinois page says phloem is where carbohydrates, or sap, are transferred up and down the tree.
To the inside of the cambium are living cells called sapwood, or xylem. The Illinois page says xylem is where water and nutrients are transferred from the root system to the photosynthetically active part of the tree, the leaves. Each spring in temperate climates, the cambium adds new xylem cells, and new wood is formed.
Penn State Extension gives the same location. It says the cambium is the region of a tree where cell division takes place that adds new wood and increases the diameter of a tree.
The spring and summer cycle
Penn State Extension says that when cell division is initiated in the spring, it proceeds rapidly, and then begins to slow down in late summer, finally stopping in the fall. The cambium is inactive during the cold winter months.
When temperatures warm in spring, the cambium is reactivated. The Penn State page says increased temperature is one of the most important factors related to the beginning of cell division in spring. It also says hormonal signals from the tips of the stems are believed to play a role in the onset of cell division in spring.
The Penn State page says some trees will stop growth before temperatures are near freezing and days that are short enough to induce dormancy. It adds that growth inhibitors created by the tree may play a role in inducing dormancy, but more research is needed to be conclusive.
Earlywood and latewood
Penn State Extension says the cambium’s yearly output creates two fairly distinct zones within each growth ring, called earlywood (or springwood) and latewood (or summerwood).
The Illinois page describes the two zones in detail. Springwood, or earlywood, is the lighter appearing ring made up of larger, thin-walled cells. Summerwood, or latewood, is the darker appearing ring made up of small, thicker-walled cells. The Penn State page says earlywood tends to be lighter in colour than latewood. Some hardwood trees such as oak and ash do not have a distinct colour variation between earlywood and latewood.
The darker colour of latewood has a cause. Penn State Extension says there will be more fibres in the latewood of hardwood trees than in the earlywood, and the latewood has a higher density than the earlywood, due to the thicker cell walls found there. Under a microscope, the earlywood portion of a ring has thin cell walls.
The Illinois page says the cumulative effect of the lighter springwood ring and the darker latewood ring represents one year’s growth for trees growing in temperate climates. Remember to count the early and the latewood portion of each growth ring as one year, not two. That is the basis for counting rings as years.
Why the count is not always exact
A growth ring usually equals one year, but the rule has exceptions. Penn State Extension says a false ring can occur when an event such as a severe drought or insect defoliation occurs. In a false ring, there are two earlywood and two latewood portions in one growth ring.
The Penn State page explains how a false ring can form. If the event is severe enough to cause the formation of latewood cells earlier than normal, and the conditions change within the same growing season so that growth resumes to normal, the tree can develop a false ring. When growth returns to normal, it can start with the rapid growth of thin-walled earlywood cells and then progress normally into latewood cells.
The Illinois page adds a warning about the outer edge of the trunk. It says the outer bark of the trees is not wood, so it should not be used to determine tree age. It also says an annual growth ring is made up of two rings, the lighter earlywood ring and the darker latewood ring, so two rings in the middle of the trunk represent one annual growth ring.
What rings can and cannot tell you
Penn State Extension says the most understood piece of information from growth rings is the age of the tree. Another piece of information is how fast the tree grew. The same page says the processes that create a growth ring are very complicated and not completely understood.
For related soil and planting topics, see the guide on how to make garden soil better and the guide on how to plant a shade perennial garden.
How we chose
The Penn State Extension article and the Illinois Extension answer are both university extension material. The Illinois page is a question and answer thread, and it is used here for the cambium and ring-count points only. Each source returned HTTP 200 when opened on 2026-10-27 with a plain request.
What we could not verify
The sources agree that rings form each year in temperate regions, but they do not give a complete model. The points below are what the pages checked do not settle.
What the sources do not say
- A complete causal model. The Penn State page says the processes are not completely understood.
- A timetable for one species. The pages checked do not give dates for spring or fall cell division in a specific tree.
- How often a false ring forms. The sources describe the event but give no rate.
Frequently asked questions
Where do growth rings come from?
From the cambium. The Penn State page says the cambium is the region where cell division adds new wood and increases the diameter of a tree.
Why are rings light and dark?
The Illinois page says springwood is the lighter ring made up of larger, thin-walled cells, while summerwood is the darker ring made up of small, thicker-walled cells.
Why does the cambium stop in winter?
The Penn State page says activity in the cambium layer of trees is inactive during the cold winter months.
Can a ring be counted as two years?
The Illinois page says one annual growth ring is made up of two rings, the earlywood ring and the latewood ring, so the pair counts as one year.
Can drought create an extra ring?
In some cases, yes. The Penn State page links false rings to severe drought or insect defoliation.
Sources
- Penn State Extension, What Exactly Are Growth Rings?: https://extension.psu.edu/what-exactly-are-growth-rings (opened 2026-10-27)
- University of Illinois Extension, Ask Extension answer on tree growth rings: https://web.extension.illinois.edu/askextension/thisQuestion.cfm?ThreadID=8436&catID=192&AskSiteID=87 (opened 2026-10-27)
