Breaking dormancy

Dormancy is a way that tree seed protects itself and remains alive until conditions are suitable for germination. If tree seeds germinated in autumn, the young seedlings would be damaged or killed by the conditions over winter. In natural conditions, the winter temperatures and high moisture in the seed help break dormancy. This can be imitated in the nursery by simply sowing seeds outdoors in autumn. However for most species, erratic germination and predation by birds and animals means this method is wasteful of time and space. Instead seed is stored over winter, but has to be given a similar temperature and moisture regime in order to break dormancy. This procedure is called pre-sowing treatment, or pretreatment.

In the past this was done by keeping the seed outdoors, as in natural conditions, but kept moist and protected from birds or animals by ‘stratification’. in most commercial nurseries this procedure has been superseded by chilling the moist seeds in fridges or cold stores. This gives better temperature and moisture control, better protection from pests, and allows the seed to be more easily checked as it approaches germination. It also permits an accelerated timescale, so for example an 18 month cold- warm-cold cycle can be reproduced in 6 months.

There is no one single method of pretreatment which is effective on all broadleaved tree species, and a variety of methods may be used. There are also alternative methods for most species, which can be used according to circumstance. Germination of all seeds in any batch, or 100% germination, is impossible; 70% is considered a success for most species. Details for each species are given in the species details.

Shallow dormancy

Some seeds, including most conifers, alder and birch, have only shallow dormancy, and an acceptable rate of germination may be achieved by simply sowing the seed direct from storage into a seed tray, as with ordinary garden seeds from a packet. Seed is very plentiful, so a low germination rate is not a problem. For higher rates of germination, use either of the following methods:

1. Place some kitchen towel in the base of a tray, and moisten it.
2. Sow dry seeds on top of the kitchen towel, and then place the tray in a polythene bag to retain moisture.
3. Chill in fridge for 3-6 weeks.
4. Bring tray into room temperature, and keep it dark by covering with newspaper or similar until seeds germinate. Check daily.
5. as soon as the seeds germinate, carefully plant them out into individual rootrainers, cells or pots.

To treat larger quantities of seed by a similar method, put the seeds into a fine net bag, made from old tights or similar, and then put the bag with the seeds inside into a container and top it up with water. Chill the seeds in the fridge for 3-6 weeks, and then remove. The seeds will be a wet mass which cannot easily be sown, but you can then put the bag containing the seeds into a spin drier (not a tumble drier), and spin the seeds dry. They can then be spread onto moist kitchen paper at room temperature as above. Keep the seeds moist and dark until they germinate.

Deep dormancy

Most broadleaved tree seeds are deeply dormant, and may have one or more of the following characteristics:

• A hard seed coat which prevents water uptake or prevents the root emerging.
• Seed coats may contain germination inhibitors.
• Embryos may be immature, and require time to mature within the seed coat.

Deep dormancy is broken by moisture, changing temperatures and time. This can be done by controlled temperature treatment, by stratification and/or scarification of hard seed coats by abrasion. The starting of these procedures must be timed so that seeds germinate in early spring, to have the benefit of a long growing season. a guide is given below, with further details of pretreatment periods given in the species details, aimed at a sowing date of about 1 March.

Controlled temperature pretreatment

There are four different combinations of temperature pretreatment. Warm pretreatment should be at 20˚C, and chilling at 2-5˚C. Details for each species are here.

• Chilling only (8-16 weeks). Begin in autumn/early winter.
• Short warm period (2-4 weeks), followed by chilling (12-20 weeks). Begin in late summer.
• Long warm period (8-16 weeks) followed by chilling (12-20 weeks). Begin in spring/summer.
• Very long warm period (40 weeks) followed by long chilling (at least 20 weeks). Begin in winter.

Chilling is most effective when seeds are chilled at the optimum moisture content – for broadleaves it is 40-60%. See Gordon, A G (1992) or Finch-Savage (1998) for details on testing moisture content.

The general procedure is as follows:

1. If the seed has been stored, it should first be soaked in water at 3-5˚c for 48 hours. change the water if it becomes discoloured during that time. Dry the seeds by draining in a sieve, air drying or by spin drying (see above), so they don’t stick together.
2. Some seeds can be treated ‘naked’, that is without mixing in compost or another medium. Naked seeds need greater attention to ensure that moisture is kept high, by being regularly sprayed with water. It’s normally easier to mix the seeds in a moist medium for the temperature treatment. Suitable mediums include half and half mixtures of peat, peat alternative or decomposed leaf litter, with a coarse material such as grit, horticultural perlite or vermiculite. Materials should normally be sterile, although there is evidence that with warm treatment, microbial and fungal activity helps break down hard seed coats.
3. Mix one volume of seed with 1-3 volumes of medium, and place in a labelled polythene bag or plastic box. Keep the container in the recommended temperature regime for the species. Chilling should be done in a fridge. Warming can be done on a shelf over a radiator or similar. Use a fridge and/or room thermometer to check that positions are suitable.
4. The seeds respire and require oxygen, so twice a week open the containers and mix the seeds thoroughly to aerate them. Remoisten as necessary with a gentle spray. Take care not to over moisten. If you can squeeze a drop out of the medium, it is moist enough. Remove any seeds that have gone mouldy.
5. As soon as seeds split or chit (begin to show a root), they must be removed and sown. Towards the end of the pretreatment period check the seeds every day. If this is not possible, sow the seeds just before the end of the treatment period, rather than risk leaving them too long. A problem is that not all the seeds of any one species or batch will chit at the same time. If stored naked, it is possible to remove chitted seeds by carefully sieving or floating the seeds, and returning the rest to cold treatment. The process is repeated until no more seeds germinate. Another technique is to wait until about 10% of the seeds germinate, which should indicate that the remaining seeds are close to germination.

Stratification

Use the following procedure:

1. Soak seeds as above, and mix with four times its weight of moist sand, which ensures that each seed is separated from any others.
2. Place in a labelled open container with drainage holes at the bottom, and protect from birds, mice and other small animals by covering with fine (6mm) wire mesh. Acorns attract mice, and it’s safest to encase the pot in fine wire mesh. plastic plant pots are suitable, or use plastic boxes or buckets with holes made in the bottom.
3. Place the containers in a shady place outdoors, or bury (plunge) in sand or gravel. Plunging maintains even cool temperatures and moisture levels. If plunged, make sure the pit is not liable to waterlogging. Inspect the containers frequently as germination time approaches. Sow seeds as soon as they split or chit.