The recommended time for planting all evergreen trees and plants, including citrus, is in spring, and the next best time to plant them is in early autumn. The question many new gardeners ask is why?
Evergreen trees and plants retain their leaves all year round. They don’t drop their leaves in autumn and become bare in winter like deciduous trees and plants do.
The reason for planting evergreen plants and trees in spring or early autumn is based on plant transpiration rate and root growth, as well as weather.
What is Transpiration?
Plants breathe through pores in their leaves that open up to take in carbon dioxide for photosynthesis, and during this process, they can also lose moisture from within the leaves.
Transpiration is the evaporative loss of water through the breathing pores (known as stomata) typically found on the undersides of the leaves. When water is lost through the leaves, the roots take up additional water from the surrounding soil to replace it.
The following factors affect the rate of transpiration:
- Temperature – higher temperatures speed up evaporation and diffusion of water from the leaf. The water holding capacity of air increases sharply as temperature increases, which leads to increased water loss from a leaf.
- Relative humidity – the relative humidity (RH) of air is the amount of water vapor it can carry. The inside of a leaf has a RH close to 100%, it’s saturated with moisture, just like the air outside on a rainy day. If the surrounding air is at a lower humidity level, this creates a gradient which causes water to move from the higher humidity leaf interior to the drier surrounding air. The lower the RH, the less moisture there is in the air, and the drier it is, which causes moisture to diffuse out of the leaf at a faster rate.
- Wind – moving air reduces the boundary layer of moisture around a leaf, that’s the still layer of water vapour around the surface of leaves. The moist, humid air around the leaves is replaced with less humid air, causing an increase in the rate of diffusion of moisture from the leaf into the surrounding air.
- Light intensity – light triggers the opening of the stomata (breathing pores on the leaf underside) to allow more carbon dioxide into the leaf for the light-dependent process of photosynthesis. When the stomata open wider, this increases carbon dioxide uptake from the surrounding air, but also allows for increased water loss through the leaf.
If the roots cannot take up water because the soil is dry, the roots themselves are damaged, or moisture is being lost through the leaves faster than the roots can replace it due to extreme hot, windy conditions, then the flow of water through the plant will be disrupted, and the plant will wilt. In citrus trees, the symptoms are easily recognisable, citrus leaves will curl along their length.
Why Evergreens Are Planted in Spring and Autumn, Not Summer or Winter
When a plant or tree is tranplanted into a new location, the biggest risk it faces to its survival is running short of water.
One way plants cope with drought conditions is by directing their energy for growth to their root system, extending the roots to reach more water.
Generally, plants grow during the warmer seasons, and in the colder seasons their growth slows down, or stops completely and they go dormant.
In spring, the weather is gradually warming up and is fairly mild, so plants are in a growth phase, and can extend roots to access more nutrients and water to fuel their growth. Having this whole season, up to three months to establish themselves, plants are better prepared to cope with the harsh conditions that summer can bring. This is therefore an ideal time to plant evergreens plants.
In summer, the weather can get quite hot and extreme. On a very hot day with strong winds, the plant’s transpiration rate can get very high. If a newly planted plant hasn’t had enough growing time to establish itself and extend its root system out sufficiently, its access to water may be limited, and in extreme hot weather, this may lead to drought stress or worse, the plant may completely dry out and perish.
Unless gardeners are extremely vigilant, providing extra water as needed and even wind protection, then a new plant may be unable to cope. In the garden nursery industry, plants and trees in pots may be watered up to three times a day during extreme heatwaves!
Technically, it’s possible to plant new plants into a garden is summer, as long as the they’re monitored daily or even more frequently, watering is kept up, and shade cloth is used as a screen to reduce wind intensity.
One of the most tragic things that occurs often is impatient consumers wanting an ‘instant garden’ spending big dollars on advanced (large grown) plants and trees, skimping on installing automated irrigation, planting or hiring a gardener to plant in mid-summer, then running off on their merry way to engage in their summer activities. Soon after a few exceptionally hot days in a row, this becomes a very expensive mistake!
Advanced plants and trees have been grown for much longer, are older, and therefore have larger canopies with more leaves, which can lose a lot more moisture. Their root systems though, are concentrated in a tiny space the size of the pot they came in, and can only access as much water as they could in the pot, until the root system grows larger to be able to reach more water. Typically, they are less resilient than younger plants with smaller canopies that have more spread out root systems relative to their canopy size.
Put simply, mid to late summer planting is only feasible if you’re prepared to babysit your plants constantly throughout the season until the heat passes and the days cool down.
Early Autumn Planting
In early autumn, the weather is still reasonably warm, so plants are still growing, but the mild weather is gradually getting cooler, which eliminates the risk of the sudden and unexpected unseasonal extremely hot days that occur in spring and punish the less resilient plants in the garden.
Any plants put in at this time have this whole season to establish themselves, and all of the following spring, up to six months of growing time before they face their first summer in the garden. Plants are more resilient the harsh summer conditions that summer by that time. This is therefore an ideal time to plant evergreens plants, and is considered the second best time to spring planting.
In winter, deciduous plants have dropped their leaves, and are dormant, they have gone to sleep for the season. Without any leaves, they don’t lose moisture. This is the peak season for buying and planting deciduous plants, shrubs, trees and vines! They’re planted in the garden, where they remain dormant until the soil warms up sufficiently in spring, then they put out new leaves and resume growth, including root growth.
On the other hand, evergreen plants keep their leaves in winter. When the leaves are exposed to bright light, the stomata (breathing pores on the leaf undersides) open, letting in carbon dioxide for photosynthesis, but losing leaf moisture also.
During winter, there’s usually more rainfall in cool and temperate climates, so any moisture from the leaves can be taken up from the damp soil, or so think many beginners, much to their detriment. It may not rain for a week in winter, and evergreen trees and plants may be battered by harsh, cold, dry winds, which can increase the rate of transpiration (water loss) drastically.
Unfortunately, in winter, plant growth slows right down, so roots can’t grow much to find more water. Additionally, soil nitrogen, the nutrient which drives leafy green vegetative growth, is fairly unavailable to plants due to low soil temperatures.
If an evergreen plant or tree is planted into the garden in winter, the root system will be no bigger than the pot, and most likely stay that way through most of winter, until the soil warms up in spring. Such a small rootball can only access the water in an area of soil the size of the pot. If it uses up the water that’s accessible by the pot-sized rootball, then it will be out of water, and begin drying out.
Technically, it’s possible to plant an evergreen plant or tree such as a citrus in winter, as long as it’s treated as if it were still planted in the pot, and watered just as often, which may be up to three times a week during dry winter periods. Evergreens won’t put on any growth during this period, so that really begs the question, why not just leave them in their pots and water them as needed throughout winter, for planting in spring?
The two most common ways that citrus trees perish are by being planted in summer or winter and then being neglected for water. Another is planting dwarf citrus in a very large pot in winter (overpotting), the root system can’t grow to take up the excess water in the potting mix, so it stays waterlogged and rots the roots. Similarly, sitting potted citrus in a pot saucer filled with water has the same unfortunate consequences.
By understanding how plants function through the seasons of the year, we can see how spring and early autumn are the optimum planting times for both evergreen and deciduous plants and trees, and why the deciduous ones can also be planted in winter without any risk whatsoever.