Planting Stock

Transplants for the planting of new forests and the restocking of harvested areas in existing forests are grown in forest nurseries.  Today some 40 million transplants are required for the annual planting programme and most of these are produced in two main Irish nurseries – Coillte and Non-So-Hardy Nurseries, with the rest being provided by a number of smaller producers and imports from abroad.

Plant production has changed significantly over the years from the time when small local nurseries were attached to forests to the highly mechanised large scale operations we see today. Transplants are produced by three main methods: bare-rooted plants, container grown plants and rooted cuttings.

Bare-rooted plants

Ireland has a mild moist climate with few extremes of heat or cold that can be experienced in continental countries. Growing conditions are favourable for bare-rooted plants and this type of planting stock is the main method of plant production.

Precision sowing of tree seed (photo D. Thompson)

As the name implies bare-rooted plants are plants free of soil attached to the roots. These are produced by sowing seed in prepared seed beds in spring and covering with a fine layer of grit to provide good conditions for germination.  The seedbeds are protected from frost and/or sun scorch by covering them with nylon mesh until such time as the seedlings are hardy enough to withstand the weather conditions in the open. During the growing season the seedlings are irrigated and fertilised as required, kept free from competing weeds and protected against insect pests and diseases.

After one and a half or two years the seedlings are lifted and lined out in rows to give them space to grow on for another year or year and a half to produce 2+1 or 1.5+1.5 transplants respectively. Alternatively, they are undercut to encourage fibrous root systems to produce 2 year undercut transplants. This latter method is generally used with fast growing broadleaved species.

Production of bare-rooted stock is a labour intensive process but machines to line out and lift plants have reduced much of the back breaking toil that nurserymen had to endure in the past.

Transplants are lifted during the dormant season and graded into size categories. They are packed in waxed cardboard boxes or co-extruded polythene bags which are designed to maintain the freshness of the plants and then shipped to the planting site.

On lifting, some plants are cold stored at around +5 degrees C for a period. This retains their dormancy and allows greater flexibility in the time they can be planted. The ability to cold store plants greatly helps contractors to better plan their work as they can now plant well into the growing season i.e. June.

Container grown plants

Container grown larch ready for planting (photo M. Keane)

Planting stock is sometimes produced in containers for direct planting into the field. Seedlings are raised in greenhouses in cells of 50-250cl depending on the species and/or size of the plants required. Optimum growing conditions (heat, light and nutrition) in the greenhouses mean that seedlings fit for planting in the field can be produced in a shorter time than transplants. This system can, therefore, be useful in making up any shortfalls in production that may occur as a result of sudden increase in demand for a species, or replacement of crops that may have been damaged by frost etc. Container plants can also be transplanted throughout the growing season as they come complete with root systems and growing media intact.

Cuttings

While the main propagation method for planting stock is by seed the nurseryman has also the option of producing plants through rooted cuttings. This method is usually used for the production of genetically improved material which may be in short supply, or for the propagation of superior clones. Cuttings are taken from stock plants grown in containers or in outdoor hedges and are usually stuck in trays which are placed in polytunnels with mist irrigation and bottom heat to encourage rooting. Once rooted, the cuttings are lined out in open nursery beds to grow on for a further year to produce C+1 transplants.

Pre-treatment of plants

 Transplants assigned to replanting sites after clearfelling sometimes receive a pre-treatment of an approved insecticide before dispatch. This is to protect the plants from the large pine weevil which is the most destructive insect pest in Irish forests. The weevil is native to Ireland where it lives in the canopy of coniferous trees and breeds in the cut stumps of trees following clearfelling. The emerging adults feed on the stems of the newly planted trees often causing severe damage, resulting in major losses in plant survival and significantly increasing re-planting costs.

While the use of insecticides in forests is controversial, efforts are being made to reduce and eventually eliminate their use, as required by forest certification.  Alternative control methods are being used such as delaying the re-planting of sites for several years (3-4) until the weevils have emerged and dispersed, or the use of a risk assessment tool to determine the likelihood of an outbreak of the pest on a particular site. Another technique that is being used with some success, but still in the research stage, is the use of nematodes that predate on the weevil larvae. However, insecticides are still consistently the most reliable method of protecting the transplant against weevil damage and, at present, are the only practical option in the event of a severe outbreak.

Past practice was to treat plants with insecticide on the planting site either by dipping the plants prior to planting or spraying them after planting.  Pre-treatment in the nursery, however, allows the application process to be carried out in a more controlled environment. Better and more consistent application is achieved, health and safety requirements are closely monitored and the disposal of surplus insecticide sludge is strictly controlled. Pre-treatment of plants also greatly reduces the amount of insecticide used on a site as only the plants that are treated.

Physiological testing

Transplant tested for root growth potential – note abundance of white roots indicating high RGP. (photo C. O’Reilly)

Although the visual characteristics of plants are important it is often their physiological (non-visual) attributes that have the greatest effect on survival and growth after planting. The visual appearance of plants may be deceiving, particularly if quality has deteriorated due to rough handling or poor storage practices after lifting in the nursery.

The quality of nursery stock can be tested or monitored by the nursery manager or the forest manager. Nursery managers often use physiological tests to determine when the stock is ready for lifting/storage, as it is only when the plants are sufficiently dormant that they can resist the stresses of lifting, handling and storage. Forest managers may be interested more in quality just prior to planting (thus also assessing effects of handling and temporary storage).

Tests used to determine physiological quality are root growth potential (RGP), root electrolyte leakage (REL), water status, and chlorophyll fluorescence (indicates integrity of photosynthetic functioning) measurements. The tests, however, only give an indication of the potential field performance of stock – actual performance cannot be predicted from the results. Nevertheless, while good quality stock may not perform well after planting, perhaps due to poor post-planting conditions, it is unlikely that poor quality stock will perform well, regardless of post-planting conditions.

For further information on the importance of physiology see:

Plant quality: what you see is not always what you get

Effect of physiological condition at the time of lifting