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