Seed stratification information

Many species are dioecious in nature, meaning that male and female flowers occur on different plants, and fruits form only on female plants. Growing these species from seed is an ideal way of ensuring that a mixture of male and female plants is obtained; if possible, grow on at least 4 or 5 plants to give yourself a good chance of having at least one male. Excess male plants can always be weeded out once it becomes clear what sex the plants are.

Details given for seeds in the tables below have a code for the recommended seed treatment prior to sowing:

Recommended seed treatments

Stratification of seeds involves mixing the seed with a moist medium and keeping warm and/or cold for a certain time before sowing. We recommend mixing seeds with moist (not wet) silver sand, using 4 parts or more sand to one of seeds; the mix should be placed in a plastic bag which can be sealed and re-opened. Label the bag well! Warm stratification means keeping the seed/sand mix at about room temperature, 60-70°F or 15-21°C; cold means keeping the mix at about 40°F (5°C) – a domestic fridge is ideal for small quantities. When cold stratifying over winter, seed/sand mixes can be placed outside in a rodent/bird-proof container (eg. a plastic dustbin). Whenever stratifying seed, check every week or two to see if germination is starting. When it does you will see white roots start to emerge from seeds, and if this happens then the seeds should be sown immediately. If this isn’t possible, keep the mix at a temperature just above freezing until you can sow.

Scarification of seeds involves softening the hard seed coat in some way to allow water to be imbibed into the seed. The simplest way of achieving this is to give the seeds a hot water soak, putting them into water at about 190°F (88°C) and allowing them to stand for several hours while the water cools. Alternatively, the seeds can be physically rubbed, eg. between two sheets of fine sandpaper – take care not to rub too much.

Dewaxing – some seeds are covered in a layer of wax (notably Myrica species) which stops the seeds imbibing water and germinating. This must be removed before stratification or sowing – the best way to do this is to rub the seeds between two sheets of coarse sandpaper (do it for periods of a few seconds at a time, then check the seeds – you only want to get rid of the wax and not damage the seeds!)

Seeds which take a long time to germinate are best sown in seed trays or pots, and covered with sand rather than compost. Very small seeds should be sown on the surface of the compost and the tray/pot kept moist by enclosing it in a plastic bag. Finally, don’t give up if seeds don’t germinate, or only a few germinate, in the first year – many seeds spread out their germination over more than one year. If the seeds are large enough, you can check their viability by cutting one in half – the seed embryo inside should be white and solid, and not soft or watery.

After the description of each species, codes are given for the recommended treatment to promote good germination. (Note that using these treatments does not guarantee germination. Seed lots vary in their requirements and these are a general guide only.) The codes used are:

  • ND Not dormant, sow in spring.
  • SI Not dormant, but must be sown immediately.
  • SC Scarify and sow in spring.
  • CS Cold stratify. Followed by a number of weeks, eg CS 13 = cold stratify 13 weeks (3 months).
  • WS Warm stratify. Followed by a number of weeks, eg WS 6 = warm stratify 6 weeks.

Warm stratification can sometimes be followed by cold stratification, eg. WS 13 + CS 13 means 13 weeks warm first then 13 weeks cold stratification.

Example: Assuming an intended sowing date of April 1st, then 4 weeks of stratification should start on 4th March, 6 weeks on 17th February, 8 weeks on 3rd February, 13 weeks on 1st January, 16 weeks on 10th December, 20 weeks on 12th November, 26 weeks on 1st October, etc. If you have trouble counting the weeks backwards there is a handy date calculator at http://www.timeanddate.com/date/dateadd.html

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