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[Germination]


                                                                GERMINATION OF CONIFER SEEDS


  

  1. INTRODUCTION


    Although growing trees from seed offers a low cost route to propagation, the motivation for producing seedlings and the long wait for useful plants derives from four main reasons:

          Seeds rarely transmit diseases

    Species marginally hardy should thrive more reliably if there is minimal disturbance to their environment.

    There are many endangered species in their native habitats

    The challenge!


    Classification of conifers has been subjected to continual change. Veitch (1881) lists 5 families, 40 genera and 343 species. Also listed are 5 ‘sub-tribes’, 33 genera and 308 species, but the lists are not mutually exclusive. The commonly accepted classification of gymnospermae has been given by Rushforth (1987) as including 3 orders (coniferales, taxales and ginkgo), 8 families, 65 genera  and about 658 species. The ginkgo and metasequoia glyptostroboides are both deciduous, the latter only discovered in China in 1941.  Lists do not include wollemia nobilis discovered in Australia as recently as 1994, or of course fossil records. Conifers include the oldest tree (pinus aristata), and arguably the oldest plant (lagarostrobus franklinii). In addition to the two above,  larix and taxodium distichum are the only other deciduous conifers. The cycadales and the gnetales, two other orders of gymnospermae, are not considered to be tree-type conifers (Mitchell). Krussmann(1983) lists 57 genera, 607 species and 2075 cultivars. Van Gelderen (1996) lists 3 orders,8 families and 62 genera.


    So there is difficulty with interpreting these differences, and of course the many thousands of cultivars increases the complexity. The RHS produces the International Conifer Register which records all claimed new varieties and establishes the name convention. The National Pinetum at Bedgebury has a collection including 10 families, 47 genera, 361 species, and over 1500 varieties. Their literature also lists 14 species ‘at risk’ growing at Bedgebury. There is clearly much opportunity to do useful work with seed germination!


    The main difficulty I have found is obtaining seeds in small quantities. It is always possible to collect seed form cones, but one is then faced with the problem of identification; something which, even with good reference books, can be well beyond the amateur. It may be necessary to have samples of seed, cones, leaves, twigs, bark, tree form and environment before identification is possible, and even then microscopic examination is often required. Purchasing small quantities of seed is often difficult, but I have found three possible sources (7,8,9).

     
  2. DORMANCY


    The time during which germination will not occur. It results from the inability of the grower to reproduce the natural habitat conditions pertaining to the type of plant and it’s own life cycle.

    There are two main types of dormancy in conifers:

    1. Mechanical-the hard seed coat prevents water and oxygen from activating or producing expansion of the seed

      Physical-embryo dormancy, resulting from inappropriate environmental conditions.


      The objective is to artificially recreate conditions to overcome the above in the shortest time.

      Much research has been done on this (Gordon ), and the main factors are temperature, humidity, light, soil composition, time of collection and storage.  All must be right to obtain the best results, but temperature is often considered to be the most important ( Chiltern Seeds ).  There is a useful guide on the web for pine, spruce, juniper and cedar ( University of Nebraska).

      Pinus aristata, the oldest living tree, cannot propagate in it’s current environment due to climate change occurring during the several thousand years of its life. The seed must be removed and subjected to conditions similar to those prevailing at the time of the original growth (actually easy to do).

       
     
  3. METHODOLOGY


    1. Scarification

      Essentially to overcome 2.1, and usually consists of carefully chipping the seed case,  soaking in water for 24 hours or both.

       
    2. Stratification

      To solve 2.2 , and requires the pre-chilling of seed in a damp environment.  In principle this simulates the ‘experience’ of a seed after it has been released from the cone.  It is often done by mixing the seed with damp sand or peat, and then leaving the seed in a refrigerator for several weeks. Recommendations for time and temperature vary, but one would expect to replicate the actual conditions likely to be experienced by the seed in it’s natural habitat.

      Gordon (1976) also points out that ‘pre-treatment in the absence of sand is almost always better and never worse than pre-treatment in sand’, although opinions differ on this.


      The current RHS surplus seed germination guide recommends an 8 week stratification for thuja, a pre-chill sow in Spring for metasequoia, but an Autumn sowing for picea,tsuga, chamaecyparis and cephalotaxus. None of these are applied to pinus or larix-it’s difficult to see the logic! Reference10 recommends soaking cedrus and juniperus cones in lye (alkaline solution, detergent) to help remove the cone stickiness. Pinus and picea genera are said to have mild internal dormancy, whereas cedrus and juniperus genera have internal and external dormancy. It is claimed that soaking seeds in a 1% citric acid solution will increase germination.

       
    3. Germination

      Opinions vary on suitable propagating media. John Innes is popular, but Chiltern seeds have found that soil-less composts like Levington is better. Sterilisation is recommended, and treatment with Cheshunt Compound or liquid copper fungicide to prevent damping off is vital.  Soil-less composts may contain re-cycled sewage, coco shell, composted bark, vermiculite, perlite or other products. It’s often difficult to find out as the bags are not marked.  I am experimenting with a peat/ sharp sand mix with added nutrients because it is easy and cheap: the recommended use of loam is difficult if sterilisation is desired, and good loam is often not readily available. There is some concern about the use of peat and the need for conservation, so there are disadvantages. Some comments about nutrients are given in the appendix. Again, interpretation  by the amateur is difficult due to variations in solubility, and lack of information about the effect of soil pH on the take up of nutrients.

       
     
  4. CONCLUSIONS


There are a number of differing recommendations, but most  confirm some form of stratification to artificially reproduce a natural habitat and stimulate germination. There seems to be differing advice on when to collect seed and how long to stratify, so experimentation seems to be the only answer. Having got the seeds started, keeping the seedlings alive is just as difficult. Great attention must be paid to avoidance of fungal infections, and providing the appropriate environmental conditions.