Transplanting in the nursery

By the end of the first year following germination in the spring, the seedlings of broadcast, small-seeded species will be crowded in the seedbed, and will need transplanting to give them more room to grow. large seeded species, including oak, cherry and chestnut, which are spaced when sown, can be left in position for another year, and undercut to stimulate root growth, although transplanting is usually preferable in small nurseries. If more than 50% of the seedlings in a bed are less than 4cm (1.5”) tall, they should be left to grow on for another year. This may happen due to late sowing or germination, or poor growing conditions.

The disturbance to the root system caused during transplanting, combined with root pruning as necessary (see below), stimulates the plant to grow more side roots so that a fibrous, bushy root system results, rather than fewer, deeper roots. This type of root system allows the plant to be dug up without too much root damage, and means it should successfully establish in its final position at the end of the second year. Transplants should have a high proportion of roots to shoots. They are described by their age and time of transplanting. For example, 1 + 1 denotes one year’s growth in the seedbed, followed by transplanting in the first winter, followed by a year ’s growth in the transplant bed. The number denotes a year ’s growth, not necessarily a full calendar year. Seedlings grown in less favourable conditions may require 2 years in the seedbed, followed by one year in the transplant bed, denoted as 2 +1.

In commercial nurseries, additional methods are used to encourage root branching. These include undercutting (see below), side cutting and wrenching, all of which can be done during the growing season, as can transplanting. The main purpose of all these treatments is to encourage root development whilst effectively checking top growth. For further details see Morgan (Forestry Commission, 1999).

Transplanting or ‘lining out’ is normally done between October and April, depending on the season and location, with autumn preferable in most areas. Seedlings lined out in autumn are usually significantly larger at the end of the following growing season than seedlings lined out in spring. In the north and east, autumn transplanting may lead to transplants being lifted by frost action, so early spring transplanting may be preferred. late spring transplanting disturbs early root growth, and plants will suffer if there is a late spring drought. An exception is holly, which is better transplanted in late spring, when the soil has warmed up.

The diagrams below show the sequence of growth from seed to 2 year old transplant. Note that the period of root inactivity is much shorter than the period of shoot inactivity. Tree roots continue to grow after leaf-fall until about mid December. Shortly before the buds open in spring, once the soil temperature exceeds 6˚centigrade, there is a burst of root cell division and expansion which sustains the plant through the spring. Once the buds burst, root growth slows. Autumn transplanting allows the plant to maximise this important early spring root growth. The same applies to the final planting out.

Lifting

Lifting and transplanting on the same day is preferable, as this lessens the chance of deterioration in storage. However, if transplanting on the same day is not possible, lifted seedlings can be stored as for final lifting, and kept for a week or so if necessary. Cold storage is commonly used on commercial nurseries to give extra flexibility in management of stock.

If possible transplant on a still, damp day, as this will reduce the drying effect on exposed roots, and lessen the stress on the plants. Avoid working in very wet conditions or when the soil is waterlogged, as root damage is likely if plants are lifted with sticky clods of soil attached. Use the following procedure for transplanting:

• Always work from the outside of the bed. insert a fork vertically to its full depth and then ease it forward to loosen the soil.
• Grasp the seedlings at ground level and lift gently, allowing excess soil to fall from the roots. The root hairs are very delicate. Don’t shake the plants or knock them against the fork to remove the soil. The soil helps protect the root hairs and lessens water loss. If working on a windy, drying day, put the seedlings immediately into a plastic bag, close the top, and take them into a shed or other sheltered place to separate, root prune and sort.
• With sharp secateurs, cut any roots longer than about 15-18cm(6-7”), which can be a nuisance when replanting. Also prune any roots that are bent or twisted from growing in cramped conditions. Such roots will stay this way, and lessen the tree’s chance of successful establishment. Root pruning stimulates side roots to grow, so encouraging the development of a bushy, fibrous root system.
  
• If there is a wide range in the height of the seedlings, then separate them into two size categories, and space them accordingly in the transplant lines. If mixed, larger seedlings will grow at the expense of smaller ones.
• The roots must be protected from drying at all times, so place the plants immediately into opaque polythene bags, and seal the tops. Keep the bags out of strong sunlight, and replant as soon as possible that day.
• Loss of moisture from the root hairs cannot be reversed by soaking the roots. This causes further damage by washing more soil from the roots.

Lining out

The amount which transplants grow in the nursery is related to the amount of space they are given. The greater the growing area, the sturdier the plant. A measurement of the root collar diameter is more significant than the height. Broadleaves for planting out in their final positions should have root collar diameters in the range shown below. This gives a guide to the size of stock you should be aiming to produce, for successful establishment.

Table 5b: Minimum root collar diameters (mm) for broadleaves: from BS 3936 (part 4)

Growth rates in the nursery will vary with species, soils, location and weather. Growth rates also depend on the genetic strain of the seedling, and on use of fertiliser, management of disease and irrigation. In commercial nurseries, most operations are mechanised, so spacing will relate to the particular make or model of machinery in use.

In non-commercial nurseries, larger spacings are usually preferable. Crowded trees tend to be ‘drawn up’, with thin stems and poor root systems. Closely packed foliage also encourages the spread of mildew and other diseases. Weak, spindly or stunted trees are much more susceptible to disease than trees which are growing freely. Wider spacing may encourage more weed growth, but this is easier to hand weed than if the transplants are very closely spaced.

If space is limited, dispose of seedlings rather than crowd them. If you have far more than you are going to need in a year ’s time, it’s better to dispose of the extra at the one year stage, rather than growing them on for another year or more, when you will find it even harder to throw them out!

The following table gives a guide to germination rates, sowing rates, seedling and distances for lining out (transplanting) at 1 year. There are no hard and fast rules about age for transplanting or lining out distances, which vary with different nurseries, areas, intended size at planting out, and personal preferences. Nurseries with less favourable growing conditions may grow seedlings for 2 years before lining out. The lining out rates suggested here are for non-commercial nurseries. In commercial nurseries the lining out rate is usually between 75 and 100 plants per square metre.

Lining out rates:

25 plants per sq m = 20 x 20 cm per plant
(rows 20 cm apart, plants 20cm apart in the row)

50 plants per sq m = 20 x 10 cm per plant
(rows 20 cm apart, plants 10 cm apart in the row)

100 plants per sq m = 10 x 10 cm per plant
(rows 10cm apart, plants 10 cm apart in the row)

Table 5c: Guide to sowing and transplanting rates for native trees

Use a measured batten or similar for spacing, as growth will be more even, and weeding much easier, if the rows and spacing are precise.

Use a dibber or trowel to plant the trees, or alternatively dig out a trench and transplant as shown below. Make sure the stem is upright, or ‘hockey stick’ stems will develop as the stem straightens up with growth. The roots should be spread symmetrically downwards from the root collar. Never ‘corkscrew’ or bend the roots into too shallow or narrow a hole, as the roots will be permanently distorted. It’s better to cut off a root that is too long, rather than bending it into a ‘j’ shape. Check the soil mark at the root collar to make sure you are transplanting to the correct depth.

Broadleaf P4 or Swellgel water-storing granules can be added at transplanting. When the tree is finally planted out, the clump of roots and water-storing granules gives the tree a good start. The granules not only release water as needed, but improve nutrient availability, and help aerate heavy or compacted soils. In addition, Broadleaf Root Dip can be used at final lifting, or granules can be added at final planting.

Undercutting

Undercutting is used to root prune without lifting the plant. This technique cuts through any long roots and stimulates the growth of a bushy root system near the soil surface which can easily be lifted for final transplanting. For undercutting in commercial nurseries, seeds are precision sown at about a quarter the density of conventional seedbeds, and undercutting is done by a tractor with a buried horizontal blade, to produce uniform results. In a small-scale nursery, undercutting may be suitable for oaks, chestnuts and cherries, which are widely spaced when sown and thus do not need spacing out by transplanting in the first winter. However, undercutting with a spade is often difficult to do without trampling on neighbouring plants, and transplanting is usually the better method for small nurseries as it combines root pruning, culling of poor seedlings and respacing.

In commercial nurseries, plants may also be ‘wrenched’ whilst remaining in the ground, which also stimulates the production of fine roots nearer the surface. This can be beneficial for broadleaves which produce substantial tap roots, including oak and ash. Wrenching regimes can be complex and are only suitable for commercial production.

To undercut, a spade can be used as shown below. Undercut in late winter, a few weeks before bud-burst. Undercutting can also be done in the growing season, but extra watering and feeding will be needed for the next few weeks, while the plants put on new root growth. Don’t undercut at the end of the growing season, in an attempt to convert long- rooted plants into suitable stock for planting out, as the plants will not have time to make sufficient new fibrous roots before they are lifted.

Push the spade under the plants as shown below. Make sure you cut to the depth shown. Practise as necessary in a spare piece of ground to get the correct angle.

Lift the plants slightly before removing the spade.