When Do Roots Grow – Spring, Summer or Fall?

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Robert Pavlis

Common advice is that tree roots grow in spring and fall and take a rest in summer. The tree is too busy in summer taking care of leaves, flowers and fruit to do much root growth. Roots also grow better in cool weather. Perennials also take a rest in cold winters, but do the roots grow in summer? This certainly seems to make a lot of sense, but is it really true?

When Do Roots Grow - Spring, Summer or Fall?
When Do Roots Grow – Spring, Summer or Fall?

Do Roots Grow Better in Fall?

A number of sources claim that roots grow better in fall but right away you have to question this advice. How do roots know that it’s October? Would fall in a cold climate not be different than fall in a warm climate? And people in the southern hemisphere have a different perspective altogether.

To really answer this question you have to know the location, or better still forget terms like spring and fall and focus on actual temperatures.

Best Temperature for Root Growth?

A study looking at potted trees of aspen, poplar and black spruce found that root elongation increased with increasing root temperature. For field grown trees “root elongation was closely correlated with soil temperature and reached maximum rates in July (Canada) for all tree species”.

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Increasing root zone temperature from 12°C to 25°C generally improves root functions related to supplying water and nutrients to shoots, leading to decreased root-to-shoot ratios, improved gas exchange, and increased chlorophyll content, which results in better root growth.

The taproot length of pecan (Carya illinoinensis), at four days of age, varied depending on soil temperature with a maximum growth at 30 °C, as shown in this diagram.

Growth of pecan taproot at various temperatures
Growth of pecan taproot at various temperatures

Cottonwood trees grow roots in summer with almost no root growth in winter. Root growth is dramatically lower in cool soil.

Cumulative root growth in Cottonwood.
Cumulative root growth in Cottonwood.

Trees have been studied more than other types of plants but the same principals apply to all plants. A soil temperature of 12-18℃ is optimal to root growth in peonies. The following diagram shows root growth of potato seedlings at various temperatures.

Root morphology and shoot growth of potato seedlings at different root zone temperatures. From Sattelmacher et al. (1990c).
Root morphology and shoot growth of potato seedlings at different root zone temperatures. From Sattelmacher et al. (1990c).

Root growth varies with species but in general roots grow best when it is reasonably warm; 20-25 °C. Root growth slows down below and above this range.

Root Growth in Black Plastic Pots and Grow Bags

Does the black material of plastic pots and grow bags get too hot and slow down root growth?

I measured summer soil temperature in both black plastic pots and black fabric grow bags. Measurements were taken a couple inches from the edge of the pot. Soil in these containers doesn’t really heat up above air temperature and it is not high enough to significantly slow root growth, at least in zone 5.

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Soil Temperature in Tropical Climates

The mean soil temperatures in Malaysia, at 1, 5 and 10cm was 25.5,  25.5 and 25.45°C, respectively. These are well within the range of temperatures preferred by roots.

Do Roots Grow Best in Fall?

Fall in colder climates has soil that is too cold to be optimal for root growth, so the common advice that roots grow best in spring and fall is wrong. The advice that roots don’t grow well in summer because it is too warm is also wrong. In cooler climates, roots grow best in summer.

Soil remains evenly warm in tropical climates so roots can grow all year long. Other factors such as moisture play a bigger role there.

Is Fall the Best Time to Plant Trees?

The false idea that roots grow best in cool soil has contributed to the the advice that fall is a great time to plant trees. Tree planting in summer is also a challenge due to high summer air temperatures and drier summer soils. The problem with fall planting in cold climates is that the soil becomes too cold for proper root growth, making spring a better option.

I discuss this more fully in Best Time To Plant Trees.

Is Fall a Good Time to Move Perennials?

Common advice: “Fall is a great time to move perennials. In fact the cooler it gets the better it is, as plants become increasingly groggy (dormant) and less susceptible to transplant stress.”

This is poor advice in cooler climates where the plant will just sit there until spring before making any roots. Fall transplanting does work for a lot of perennials because they are tough plants but it is not an ideal time. Moving in summer is also not good for them. In cold climates it is much better to move plants in spring.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

19 thoughts on “When Do Roots Grow – Spring, Summer or Fall?”

  1. There are so many variables that affect root growth that it is very difficult for the average gardener/landscaper to really know what is best.

    For example, the type of soil has a significant impact on root growth. Most residential and many commercial plantings are in disturbed or altered soils.

    The location of the tree/plant in the landscape can make a large difference. Sometimes just a few feet cam make a difference between the growth of identical plants, planted at the same time, by the same person, in the same manner.

    Also, micro climates exist within all climates with just a mile or two making a difference in growth and/or whether the same plant/tree species will thrive or even die.

    Then there are individual differences between plants, just as there are with humans.

    My personal experiences in Central Ohio (Columbus Area) several years back were that the best time to either plant a tree/shrub/perennial grass was in late October up to the light freezing of the soil in December. When a landscaper that was in business in that area for over 25 years told me that, I did not believe her. I fact, I was dumbfounded (in addition to being just dumb). However, I tried it once and saw a remarkable difference in how the plant grew the next year beginning in the spring. Every year after that, I always planted in November to early December with the same results. As an amateur, I attributed it to a “setting of the biological clock” for the roots, which the plants seemed to love thus allowing them to get “off to the races” at the first sign of spring.

    As an amateur, I don’t think enough work has been done on plant root systems or on the metamorphosis of plants.

    Thanks for a great article!

    Michael Upper

    Reply
  2. Excellent. As an aside, I am on an extended visit to sri Lanka. At lower altitudes it is always hot and oppressive. over 30 almost every day. And anything you drop on the ground roots rapidly and grows with amazing speed. So “fall” is nonsense.

    Reply
    • That is a good question and I tried to find the answer before posting the blog post – but I am still looking for facts.

      Common advice is to move in fall. However, we routinely also move in spring. I am not sure if fall is really better, or it is just what people do.

      Reply
  3. By the way, just for general interest, Kentucky has soil temperature gauges throughout the state. When I look at the records for this past winter, the coldest at 4″ deep was 40° F (4.5° C.) and the average looked to be about 45° F (7.2° C.)

    Reply
  4. I get this question—-“I bought a potted shrub in October, when should I plant it?” I answer to plant it right away. My reasoning is that the root ball is better off in the soil than in a pot in the garage. Is that the right way to think about this?

    Reply
    • It is better in the soil than the garage. You can also just sink the pot in the ground and then plant in spring. This might be better if you end up doing some root washing or pruning.

      Reply
  5. Bravo for the concise and easy-to-understand explanation. “Know where you ARE.” Know your climate AND your plants.

    I’m always curious about the sources of these myths.

    Reply
  6. What you have stated matches my observations of plant growth here in the NW of England.
    Having said that, soil temperatures @10-15cm depth here have a relatively narrow spread, rarely climbing above 20°C or falling below 4°C.

    Reply

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