Finnish Forest Research Institute  Metla

Media release 16.9.2005

Dissertation: Extension of the planting period of Norway spruce container seedlings: risks related to the drought – growth stage dynamics and handling practices

In the doctoral dissertation of Pekka Helenius Norway spruce container seedlings were studied to ascertain whether the planting could be postponed until July without a significant increase in mortality due to drought. The study also included investigation of risks related to thawing and planting of seedlings that were stored frozen in a nursery’s refrigerated cold store over winter, and the impact of the risks on survival and growth when the planting period is extended from spring to summer. The results of this dissertation submitted to the University of Helsinki indicate that actively growing, well-watered Norway spruce container seedlings can safely be planted as late as in July, provided that the soil is not exceptionally dry.

Most of the Norway spruce seedlings grown in Finnish nurseries for forest farming (approx. 90 million) are planted during May and June. Additionally, during the same period more than ten million Norway spruce seedlings imported from Sweden are planted. The early and short planting period has been justified by explaining that seedlings profit from soil moisture in spring. Indeed, the wet spring conditions represented an important factor behind the successful forest regeneration in the 1970s and early -80s, when most of the seedlings delivered for forest cultivation were bare root seedlings sensitive to drought. Container seedlings, which are better protected from drought, were commonly adopted in forestry after the mid-1980s, but no changes have been made to the recommendations concerning the planting period. At the moment, almost 95% of all Norway spruce seedlings delivered for forest cultivation are container seedlings. In early spring soil is often too cold for the seedlings to root and absorb water. The short planting period also leads to bottlenecks both at nurseries and in the organizations responsible for the planting.

The material for the dissertation was collected between 1999 and 2003 in experiments performed in the greenhouse, nursery field and a nearby clear cutting area of Metla’s Suonenjoki Research Station. The testing was designed to simulate conditions such as seedlings being exposed to drought during transportation and intermediate storage, and to various levels and lengths of drought (1-6 weeks) following planting at the turn of June-July. When planted, some of the seedlings were already actively growing while some were still dormant. The dormancy was continued by keeping the seedlings stored in a frozen state over winter for a period longer than normal. At the same time, the effect of the prolonged frozen storage state on the seedlings’ stored carbohydrates was also studied. Drought stress was imposed during three growth periods in different weather conditions (1999, 2000 and 2002). The frozen-stored seedlings were thawed and kept in intermediate storage for different time periods and at different temperatures before planting during 2001 and 2002.

Height and root growth was reduced in the seedlings that had been planted in an actively growing state at the turn of June-July and exposed to post-planting drought. The seedlings’ height and root growth was slowed down and mortality increased not only by drought but also by pre-planting drying of plugs and seedlings, when the water content of the plug was reduced below 20% (vol.). Strong variation was observed in drought conditions and evaporation from soil caused by annual variation in weather conditions, which caused considerable variation in plant survival and growth during the drought periods. Regardless of the year, however, only when the dry period in June-July was longer than two weeks, was there a significant effect on mortality of well-watered seedlings that had been planted in an actively growing state.

Extending the frozen-stored period till the end of June (34 weeks) did not noticeably decrease the carbohydrate content of the seedlings’ needles or weaken the seedlings’ successful field performance. Unlike the seedlings of the same batch planted in active growth state and taken out of the frozen-stored conditions already after 30 weeks, the drought treatment did not have an effect on the rooting and chlorophyll fluorescence of the seedlings planted in dormant state at the end of June. The drought periods also had a smaller effect on the water potential of the seedlings planted in a dormant state. However, the plants that were planted in an active growth state rooted better than the dormant ones, except when imposed to the longest drought periods (i.e. 3 weeks). In other words, the drought-resistance of seedlings planted in dormant state is manifested only after exceptionally long drought periods. When frozen-stored seedlings are planted without thawing, it can hamper growth and increase mortality regardless of temperature, especially if the soil turns dry during the rooting stage. For thawing of root plugs and successful field performance the safe thawing times and temperatures were 4-8 days and 9-12 ºC.

Publication: Helenius, P. 2005. Extension of the planting period of Norway spruce container seedlings: risks related to the drought – growth stage dynamics and handling practicesUniversity of Helsinki, Department of Forest Ecology. Dissertationes Forestales 3.

For additional information, please contact:

Researcher Pekka Helenius, Metla/Suonenjoki Research Station, tel. +358 10 211 4868, pekka.helenius @

Press releases in English are available at:   Metlan RSS-newsfeed
Finnish Forest Research Institute - Metla/Communications
Unioninkatu 40 A, FI-00170 HELSINKI, FINLAND. Phone +358 10 211 2000    E-mail: info @