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FAQs

What seedling stocktype should I order?

How can I monitor seedling temperature prior to planting?

How deep should my container seedlings be planted?

Are my seedlings ready for freezer storage?

What are the benefits of secondary needle pine?

Is it desirable to plant near old stumps?

Still have questions? Please contact a Customer Support Representative at a PRT nursery near you. 

What seedling stocktype should I order?

Choosing the correct type and size of seedling to plant (i.e. stocktype selection), has a major influence on the establishment and early growth of forest plantations. This in turn affects your achievement of free growing obligations and green-up requirements, along with future management decisions. These optimum seedlings minimize overall reforestation costs while achieving established goals for initial survival and growth.

Stocktype selection can be broken to the following phases, which are further described below:

  • Identify the site ecotype and select the speciesThe basic unit of biological classification. ‘Fir’ and ‘pine’ are two examples of species classifications. and seed sourceThis covers all material from a single tree to any collection of trees within a region of provenance or seed zone.
  • Determine the planting window
  • Assess the specific site limiting factors
  • Review available site preparationAny of a number of methods used to modify planting sites to improve the ability of seedlings to establish and grow. Common methods include burning, mounding, disc trenching and piling. In the US, herbicides are commonly used for site preparation. methods to alleviate the limiting factors
  • Determine stocktype best suited to the site
  • Compare cost of various site treatment and stocktype combinations
  • Select the optimum seedling

Identify site ecotype and select species and seed source
Identifying the site ecotype (e.g. nutrients, soil, moisture, etc.) and selecting the appropriate species and seed source are generally done early on in the process, prior to logging. In most cases, your choices at this stage are limited and dictated by site and management factors.

Determine the planting window
After the desired species and seed source has been chosen, the planting window must be determined. This will dictate the choice of frozen stored/over-wintered stock for spring plantingShort form: SP
Spring delivery products are lifted in the late fall, packaged and stored at –2°C/18°F over the winter. This allows for a wide delivery window in the spring, enabling the seedlings to be planted when field conditions are optimal, typically from mid-April to mid-June.
, or hot-lift/current crop stock for summer plantingShort form: SU
Summer delivery products are lifted in the summer, packaged and shipped immediately to the customer for out planting. Seedlings are morphologically active and it is recommended they be planted within three days of being delivered from the nursery. Typical summer planting dates are from June 20 to late July.
. If the planting sites have spring access, the ground never freezes or thaws early, and the probability of late spring frosts are low, then spring planting can begin as soon as feasible and continue until late June. For sites with late spring access (e.g. high elevationShort form: Elev
Seed collection elevation.
 cut blocks), hot-lift stock can be planted from late June until late July, as long as the site is not subject to summer drought. When selecting a planting window, operational logistics will play a role, but they should never be allowed to compromise the seedling's biological requirements.

The planting windows above apply primarily to interior boreal areas. For coastal reforestation sites, other options are available such as planting during the late summer or early fall if the areas receive sufficient precipitation.

Assess site-specific factors
The next step is to assess the factors that limit plant growth on the site or in the general area if it is not feasible to order stocktypes for each planting site. Common site limiting factors and appropriate planting stock for these conditions include:

  • vegetation competition – select large stock
  • snow/vegetation press – select large, sturdy, well branched stock
  • cold soils – select stock grown in shorter containers
  • drought – select smaller, well balanced stock
  • shallow soils – select stock grown in shorter containers
  • frost – no stocktype can fully avoid frost damage, although species differ in tolerance of frost (e.g. lodgepole pine will withstand frost much better than interior spruce)
  • wet soils – no stocktype can be used to alleviate wet planting spots. In fact, the use of mechanical site preparation may be your only option for enhancing establishment and growth on very wet sites.

Review available site preparation methods
After the site limiting factors have been assessed, available site preparation methods to alleviate the growth limiting factors should be examined. Examples include tools such as chemical and mechanical site preparation, the use of which is usually governed by environmental and/or monetary factors.

Determine stocktype best suited to the site
Taking into account the use of any planned site preparation, the stocktype best suited to the site's limiting factors needs to be determined. Keep in mind that there are limits as to what any stocktype can overcome. Under favorable conditions, large seedlings generally grow larger in absolute terms than comparable smaller seedlings. They do not have any additional physiological attributes to enhance performance, but simply occupy a greater area within the planting spot. A greater photosynthetic area equals quicker growth and access to the site's resources. This is coupled with a greater number of branches and buds which equals more places for new shoot growth. Once site-limiting factors (e.g. vegetation competition) are overcome, small and large stock will both grow at the same rate governed by the site. However, where moisture is limited, large stock often does not survive as well as smaller stock due to the greater transpirational demand. Also, on sites with minimal or no vegetation competition, initial stock size differences are not as important as the large stock does not offer as much of an initial advantage over the smaller stock.

A clear understanding of the various types of nursery stock available will allow you to make the most productive stock selection decision. Current Canadian nursery stock is grown mainly in containers, replacing field grown barerootShort form: BR
Seedling crops grown in field beds and harvested without soil or “bareroot”.
 seedlings.

Container seedlings may be over-wintered/frozen stored or hot-lifted during the summer. From a nursery perspective, the ideal container is one in which the nursery can economically, consistently and reliably grow seedlings of the desired size. From a field perspective, the ideal container is one that produces seedlings of a desired size capable of vigorous cost-effective growth after planting.

There are basically two container types:

  • rigid-walled containers which are removed before planting exposing a firm root plug (e.g. Styroblock, Multi-pot)
  • soft-walled containers which remain intact at planting (e.g. Jiffy).

In Canada, the Styroblock has been in use for over 30 years, and is the most commonly used container.

After container type and size, stocktype is primarily characterized by: i) height; and ii) root collar diameter (RCDRoot collar diameter.). Also, dividing height by RCD gives you the height-to-diameter ratio, which is an indicator of seedling sturdiness. Usually, seedling specifications including minimum, target and maximum height, along with minimum and target RCD are included in the seedling contract, which is drafted when ordering seedlings. In the nursery, there are biological limitations as to what can be produced in any given container size for each species. Optimum seedling specifications reflecting what can be reliably grown in the various container sizes in a cost-effective manner have been developed. The nursery grows to the target specifications of the particular stocktype, but there will always be variation as seedlings are biological organisms, and the distribution of their attributes will tend to fit the familiar bell curve.

Stocktype selection should be based on target seedling specifications that can be reliably achieved within a particular container size. If a larger seedling is desired, a larger container size with correspondingly larger height and RCD specifications should be chosen. In essence, after assessing the site's limiting factors and determining what size stock you require, you would order stock in the container size that has target seedling specifications which match your previously determined stock size requirement.

Compare cost of various site treatment and stocktype combinations
After the stocktype best suited to the site has been determined, then the cost- effectiveness of the various available site treatment/stock combinations must be compared. This is the final step in determining your optimum seedling. To compare effectiveness we need survival and initial growth goals, which in some cases have already been set by stocking and free growing height requirements. Analysis of regeneration costs requires evaluation of these goals and cost in relation to site preparation, stocktype, and post-planting vegetation control. At this point, you must remember to consider total establishment costs – initial savings may result in additional costs later (e.g. what is the cost of fill-planting?). Alternatively, initial investments in nursery culture may reduce overall costs in the long run, through a reduction in subsequent silvicultural treatments.

Select the optimum seedling
Finally, rank the silvicultural treatment/stocktype combinations by total cost per hectare. Which combination meets your free growing requirements at the lowest cost? This is the difficult part, as we usually do not have all the data we would like. Often it comes down to the level of risk you are comfortable with. Nonetheless, after comparing the above, you will be in a better position to choose your optimum seedling or stocktype. 

Still have questions? Please contact a Customer Support Representative at a PRT nursery near you.

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How can I monitor seedling temperature prior to planting?

Small, user friendly, inexpensive temperature data loggers with numerous applications for seedling/reforestation practices make environmental monitoring a breeze. Possible applications include:

  • monitoring the temperature environment from the nursery to planting hole
  • comparing soil temperature differences on mechanically site prepared vs. raw planted ground
  • comparing temperature difference in brushed vs. non-brushed sites
  • monitoring damaging frost events

Several companies manufacture data loggers. At present the best bang for your buck is the Thermochron iButton from Dallas Semiconductor Corporation. It is a single-chip time and temperature logging device capable of recording temperatures between –10°C (14°F) to 85°C (185°F) for more than 10 years or up to 1 million measurements, whichever comes first.

The iButton is armored in 16 mm stainless steel, submersible, and dirt- and impact-resistant. The current price is approximately $10 US per unit, depending on the quantity purchased. A Thermochron iButton evaluation kit containing a DS1921 iButton, Blue Dot receptor for downloading the data, 9-pin Universal 1-Wire computer port adapter, and mounting software is available for $25 US.

Software can be downloaded free of charge from their website. The data can then be downloaded into a spreadsheet or right onto its own web page, as each iButton is web addressable. For more information, visit the iButton website.

Still have questions? Please contact a Customer Support Representative at a PRT nursery near you.

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How deep should my container seedlings be planted?

Early research against planting too deeply, going as far as to call deep planting “the greatest hazard to plantation survival.” However, more recent literature recommends deeper planting. Like all aspects of forest regeneration, planting methods must be looked at on a site by site, and species basis.

The recent research is mostly based on southern pines, grown in hot, dry conditions where more deeply planted seedlings may benefit from increased stability, drought avoidance and root collar (i.e. basal stem) protection from heat damage. Deep planting is not recommended on wet or poorly drained sites. However, in planting spots that have been mechanically site prepared (e.g. excavator mounds) the seedling plug is best planted an inch or so below the soil surface, if it is expected that some settling will take place. In general, "planting check" will be reduced by placing seedling roots at a depth favorable for their growth, such as warm, moist, well drained, nutrient rich substrates.

When talking about planting depth at the planting spot level, your point of reference must be taken into account. For example, are you talking about screefing away the duff and planting into the mineral soil, or planting directly into the forest floor on naturally raised micro sites?

In northern areas of western Canada, the past operational standard has been the "two finger" rule (i.e. planting the seedling approximately two fingers below the top of the seedling plug). This was developed in response to concern regarding poor root-soil contact leading to moisture stress, as well as fears that the plug would wick out all of its moisture if exposed. Although there will be surface drying of the peat if the top of the plug is left exposed, forest nursery peat has large, non-capillary pores which impede the upward movement of water, thereby preventing wicking. The "two finger" rule is gradually being abandoned in appropriate northern regions, and seedlings are now being planted with the plug level with the top of the fermenting/humus layers of the forest floor, often leaving the upper litter layer intact.

When certain species such as black spruce are planted deep, adventitious roots will form on the buried stem, branches, and root collar. Although an adequate root system will develop as the adventitious root supplement or in some cases supplant the original roots, field performanceAbility of trees to perform their intended function on a reforestation site. will be slow until this takes place.

Other species such as pine do not commonly form adventitious roots, thus deep planting in cold, wet soils is even more of a concern. Stocktype, especially plug length is another important consideration when considering planting depth.

On cold, wet sites with no mechanical site preparation, shorter plugs such as the Styroblock PSB 410 work well, as they place more of the roots in the upper soil profile where root growth will occur because of the warmer, more favorable conditions. On mineral mounds, longer plugs such as the PSB 415B are often used so that the plug is closer to nutrients and moisture in the underlying duff sandwich. 
Still have questions? Please contact a contact a Customer Support Representative at a PRT nursery near you.

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Are my seedlings ready for freezer storage?

The date your seedlings are lifted and put in freezer storage is critical to their subsequent outplanting success. At PRT, we closely monitor seedling development and lift for physiology rather than operational reasons. As in any nursery, practical business considerations play a role, but never at the expense of seedling quality. But just how can you tell if a seedling is physiologically ready to be lifted? To get to that answer, an understanding of dormancy in conifer seedlings is first requirement. The dormancy cycle in temperate zone, determinate growth, conifer buds occurs in three stages:

Predormancy quiescence, where growth is controlled by the environment (e.g. photoperiod and water stress).

  1. Dormancy or rest, where growth is controlled by the physiology of the bud.
  2. Post-dormancy quiescence, where the bud will flush after obtaining its species/genotype specific required amount of heat sums.

The seedling moves from dormancy to quiescence after accumulation of several hundred hours of low temperatures, zero to 10°C (32°F to 50°F). This chilling is required before buds can break in response to warmer temperatures. If not totally fulfilled at the time of lifting, the chilling requirement may also be fulfilled by cold storageStorage of trees for winter and spring planting at temperatures above freezing, typically at 2°C (36°F)., although temperatures in freezer storage at -2°C (28°F) are considered below the optimum range to accumulate chilling.

To withstand freezer storage, it should be obvious that seedlings need at least a certain degree of cold hardiness. In the past, poor outplanting performance has been linked to seedlings lifted too early in the fall, probably because they had not reached a sufficient physiological state to endure the stresses of freezer storage. The increase in the seedlings’ relative stress resistance enables them to tolerate mechanical handling, loss of roots and fluctuations in moisture, temperature and other stresses during lifting and storage.

Seedling stress resistance peaks in the post-dormancy quiescence stage in mid-winter. For example, the best freezer storage of black spruce was accomplished using seedlings that were near the end of dormancy period and approaching a quiescent state.

The intensity of dormancy can be reliably estimated by measuring the time for terminal buds to break in a forcing environment (i.e. days to bud break). In the simplest terms, the longer it takes for buds to break, the greater the level of dormancy, although as mentioned peak classical dormancy is not necessarily the point of greatest stress resistance. Dormancy intensity weakens as chilling units accumulate, thus if seedlings are put into a forcing environment, days to bud break (DBB) will decrease the later in the year it is tested. As well, substantial differences in DBB between Spruce seedlots suggest that in addition to chilling requirement differences, there are differences in the number of heat sums required to break bud. Although valuable in research, measuring DBB is seldom done on an operational basis because of the time involved to obtain results, hindering its usefulness as an operational management tool.

Root growth potential (RGP) often does not correlate with root growth after planting, but has been related to DBB along with bud dormancy, cold hardiness, and stress resistance. Variation in RGP is related to the shoot dormancy cycle. It is typically low in the summer, rises in the fall, peaks in the winter, and falls to a low level as bud break approaches. The midwinter increases coincide with the accumulation of chilling hours, with the peak representing the fulfillment of this requirement. As it is desirable to have high RGP at the time of planting, it has been suggested that it be high going into storage. Normally RGP will remain high in cold storage, but is more likely to decrease than increase. However, as RGP can change in cold storage in response to the accumulation of further chilling units, seedlings do not necessarily have to have high RGP values to be safely lifted.

As well, testing seedling RGP just after lifting may never become a useful way to assess planting stock quality, because any meaningful interpretation of the results would have to depend on a specific knowledge of the seasonal patterns of the various seed sources in the nursery. The limited value of RGP as a guide to the storability of tree seedlings has also been noted by others. Fortunately, as the seedling progresses through its annual growth cycle, RGP, bud dormancy and cold hardiness change according to a regular pattern. By knowing the relationships between the various attributes, one can use a quickly measurable test such as cold hardiness to estimate the others.

In western Canada, seedlings are judged to be ready for freezer storage when they are cold hardy to -18°C (-0.4°F). This temperature is much greater than the actual storage temperature of -2°C (28°F), but earlier research found that at this temperature (i.e. just before the RGP reached its peak), RGP was maintained or increased during freezer storage at -2°C (28°F).

PRT has a comprehensive quality assurance program in place for all of its seedlings. Part of this program includes frost hardiness testing, to determine when seedlings can be safely lifted. Growers collect seedlings randomly from Styroblocks in representative growing areas in the nursery. Seedlings are then subjected to freezing temperatures (i.e. -18°C (-0.4°F) in a programmable freezer, after which they are placed in a greenhouseShort form: GH
Polyethylene film or glass-covered structure, complete with heat and ventilation.
 or growth chamber and frost injury/damage is assessed visually. Low incidence of injury represents high frost tolerance. The B.C. Ministry of Forests has now replaced visual frost injury assessment with chlorophyll fluorescence measurement.

In addition to prestorage assessment, PRT removes samples of each seedlot from storage well in advance of shipment, pots them up and flushes them out in a greenhouse to assure their viability. As demonstrated on various species in western Oregon and northern California, RGP tests run just after lifting should not be used to predict planting stock quality after cold storage.

In response to the question of whether your seedlings are being lifted at the right time, several tests are theoretically available, but as with any stock quality assessment program, a balance between the desirable, the affordable and the necessary will have to be made. According to the scientific literature, although increased testing may provide greater insight for research purposes, our current procedures adequately assess seedling readiness for frozen storageStorage of trees for winter and spring planting at temperatures below freezing (–2°C/28°F).. It is in everyone’s best interest that seedlings are lifted at the optimum time, as we all want to enhance field performance after planting.

Still have questions? Please contact a contact a Customer Support Representative at a PRT nursery near you.

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What are the benefits of secondary needle pine?

The first formed needles on pine are called primary needles, which on lodgepole pine may continue to be produced until the end of the first growing season. They are usually not found after the second growing season. Normally starting in the second year, mature or secondary needles are formed (i.e. fascicle needles). Sometimes they occur naturally during the first year, usually at the end of the season. Secondary needles may be induced in the nursery most commonly through the use of long photoperiods, but also with nutrients and warm temperatures. Genetics also plays a role, as seedlot differences in the secondary needle formation are often seen in the nursery.

Secondary needle pine is often considered more resistant to stress such as drought. One reason may be because it looks more like a mature tree, although primary needle pine has less foliage and thus less transpirational area. One area of concern with primary needle pine is that the needles may solarize (i.e. burn off) after planting the first season, which makes the tree look really ugly. It appears that secondary needles do not solarize as much.

As well, secondary needle pine has a greater percentage of dry weight in foliage, and also usually higher nutrient concentrations, which translates into higher nutrient contents in the shoot. Finally, secondary needle pine has a whorl of buds at the apex vs. a single terminal bud found in primary needle pine. The multiple buds present on secondary needle pine may be advantageous on frost or browse prone sites, as they offer a backup if the terminal bud is damaged.

At present, there is no significant evidence in the literature to support enhanced field performance of one needle type over the other. From the scant research published to date, results are variable. Work on Scots pine has shown that primary needle pine has a greater shoot growth potential after planting because of increased stem units in the bud.

Steve Omi in Idaho reported greater root growth potential in primary needle pine (N.B. His photoperiod was extended later in the year than B.C. operational methods). He also reported greater early frost tolerance in primary needle pine, which evened out by the lift. Chris Hawkins (formerly of the BCMOF Red Rock Research Station in Prince George) initiated a trial to examine the effect of needle type on field performance. His early results suggested little advantages to the secondary needle pine.

Nursery culture and environment (i.e. greenhouse vs. outdoor compound-grown) most likely will also have an effect on field performance. For example, a seedling grown under cover in a greenhouse will most likely be different than one grown outdoors from day one in an open compoundShort form: OC
Outdoor growing area.
 in the Okanagan. As well, season of planting should also be considered, as on spring plant primary needle pine, it will have secondary needles within a couple of weeks after the new flush. It is worth noting that hundreds of millions of primary needle pine have been planted with apparently very favorable results. In fact, because of research in the early eighties that reported lesser primordia in the buds of primary vs. secondary needle pine, secondary needle container-grown pine in B.C. was considered undesirable and culled.

Still have questions? Please contact a contact a Customer Support Representative at a PRT nursery near you.

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Is it desirable to plant near old stumps?

Any planting locationGeographic area where seed was collected, often referring to the name of an area or creek drainage. questions should be answered with the specific site and its limiting factors in mind. In the northern boreal forest, however, there are several advantages to planting near old stumps.

Firstly, they provide a naturally raised microsite, resulting in plug placement into warmer, nutrient rich substrates. Stumps reradiate heat at night, possibly providing some protection against frost. By planting on the appropriate side, stumps provide physical barriers (i.e. obstacle planting) against vegetation press, snow creep and animal (e.g. cattle) trampling. As well, some people say that certain types of mycorrhizae can survive for up to one year on stumps. On hot and dry sites, stumps can be used to provide shade to newly planted seedlings.

Of debate is the distance from the stump that the seedling should be planted. It depends to some degree on stump size, as larger stumps tend to have unsuitable planting substrates such as large air pockets at their base. Two other possible areas of concern when planting next to stumps are later root asymmetry problems resulting in instability leading to toppling and increased incidence of root rots.

Still have questions? Please contact a Customer Support Representative at a PRT nursery near you.

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