Planting stock

How trees grow

There are a few points relating to tree growth which are useful to remember when planting and caring for trees.

• The major roots of trees last for the lifetime of the tree, and become thicker, in the same way that branches do. They spread out in a shallow network that is mainly within the top metre of soil, with occasional deeper roots called sinkers. Often, though not always, there is a tap root below the tree’s trunk.
• The main roots bear masses of smaller roots, which, like the leaves in the tree’s crown, have only a short life. Each separate root hair explores a patch of soil, extracts water and nutrients from it, and then withers away. Only those just behind the root-tip are active, and so continued growth of the tree depends on the constant growth of the fine, white new roots.
• When you plant a tree in autumn, its roots will start growth at once, using nutrients stored in the stem, though its shoots remain stationary until spring. Root growth stops about mid December. There is a burst of new feeding roots early in March, as the soil warms to 5˚C, and before the aerial parts show any growth. Root growth then slows during the main period of leaf and shoot growth. This is why autumn planted trees have such an advantage over spring planted trees, as they have made some root extension in autumn, and are able to maximise the important early spring growth of root hairs.
• Root hairs extract water from the soil by osmosis. The hairs contain sap with dissolved nutrients at greater concentration than the salts in normal soil-moisture. The root hairs are semi-permeable membranes through which the soil moisture can pass, and be drawn up into the plant. If the soil moisture has a high proportion of dissolved fertiliser or salt, water will pass the other way, and the root hairs will be ‘scorched’. Water cannot be absorbed through main roots, but only through root hairs.
• Roots of most species require free oxygen in the soil. However, common alder, grey sallow and some other species can survive in wet conditions by making use of oxygenated fresh water.

Size of planting stock

Trees transplant best when they are small and young. Younger trees have the following advantages:

• Because they have a better root to shoot ratio than larger stock, they more easily overcome the shock of transplanting and quickly put on new root growth.
• They are easier and cheaper to transport and handle, and less prone to physical damage or drying out when in transit.
• They can be notch planted rather than pit planted (see below), which is much quicker and just as effective.
• There is less requirement for staking.
• Being grown for only a short time in the nursery, they are cheaper to buy or less time-consuming to grow yourself.

Tree shelters or guards are commonly used with small transplanted trees. Shelters or guards add to the expense of planting, but have a multiple use in enhancing growth, protecting the tree from browsing animals, acting as a marker so that small trees can easily be located, functioning as a stake to keep the tree upright, and allowing the efficient use of herbicide spray (see below).

There are two types of stock generally used for woodland creation and other native tree planting schemes. These are bare-root stock and cell-grown stock.

Bare-root stock

Bare-root stock for woodland planting is normally 1-3 years old. These plants are grown in open ground in a nursery, and lifted in late autumn when growth is dormant, for final planting out. They can be described by their height, and/or by the age and method of growing. A tree described as 1 + 1 has been grown for one year and then transplanted in the nursery and grown for another year. 1 u 1 describes stock which has been grown for one year, undercut, and then grown for another year. The reason for moving or undercutting in the nursery is to promote the growth of a fibrous, branching root system, which will sustain the tree when it is planted out. Plants which are grown for two or three years without this disturbance tend to produce fewer, deeper roots, which are awkward to dig up and transport, and which are slow to produce new growth after final planting.

The quality of bare-root stock should be judged by the volume of fibrous roots, and by the root collar diameter, rather than by the height of the plant. The root collar diameter is a good measure of the sturdiness of the plant, and its ability to thrive after transplanting.

Table 6a: MINIMUM ROOT COLLAR DIAMETERS (mm) FOR BROADLEAVES (from BS 3936: Part 4)

Table 6b: STOCK TYPES SUITABLE FOR WOODLAND PLANTING

Cell-grown stock

Cell-grown or rootrainer trees are one-year-old seedlings, grown in special tall, narrow pots designed for tree seedlings, which promote good downward root growth without spiralling. Once roots start to grow spirally inside a pot, they tend to continue growing spirally even when planted out. The plants are normally grown for one season only in the cell or rootrainer, before being planted out in the autumn.

The diagrams show the development of root systems in a rootrainer.

Cell-grown plants are more expensive than equivalent age bare root seedling trees, and are more bulky to transport. However, cell-grown stock has the advantage that the roots are not disturbed when planting out. Water-retaining granules, fertiliser or mycorrhiza can be included in the growing medium. Because the roots are protected, cell-grown plants can be planted before they become fully dormant, while they are still in leaf. They should put on some root growth in the autumn after planting, and then grow away quickly in the spring, putting on more height growth than similar age bare-root plants. The ability to plant in early autumn reduces the pressure to get all planting done in the late autumn/early winter period. Cell-grown plants can be planted between July and December, with September/October normally the best time, depending on season and location. Planting in July and August should only be done where the plants can be regularly watered.

Cell-grown plants are grown in polytunnels or glasshouses, and must be moved outside in the nursery for hardening off several weeks before they are due to be planted. If there is a delay between delivery and planting out, store the plants in a shady, sheltered place, and keep watered while in leaf. If storage is required during the dormant season, keep the plants in a frost-free place, or cover at night with horticultural fleece or similar to prevent the containers freezing. Planting should not be delayed until the spring, as the plants will deteriorate, and the advantage of using cell-grown stock will be lost.

All transplanted trees benefit from weeding (see below), but for cell-grown stock thorough weeding is absolutely essential. The plants are very small and easily swamped by weed growth, and although they have a good root to shoot ratio, their small size means that roots are also small. Cell-grown stock must be nurtured during the year after planting, by being regularly checked for weed growth and watered in dry spells. Because of their small size, cell-grown plants have the most to gain from the use of tree shelters.

Provenance

The provenance of a plant is the place from where the seed or cutting it was grown from was originally collected.

Plants of the same species vary in their adaptations to local conditions, shown by differences in flowering time, flower or leaf size, form of growth and other characteristics. They also vary in the range of other organisms which they are able to support.

In the past, trees grown in British tree nurseries tended to be from seed of trees selected for their form and timber quality, often collected originally from trees on the continent. These may lack the special adaptations for local conditions, and the ability to support a wide range of organisms. By not perpetuating plants of local provenance, particular genetic strains may also be lost for ever.

Identifying trees of local provenance can be difficult, as trees have been widely planted for over 400 years. However, reproductive material is considered to be of local provenance if it is from stands of trees which appear well adapted to the location in which they are growing. Many nurseries in Britain now stock trees and other plants of native and local provenance.