What is Stratification?
Some seeds such as Aquilegia and Larkspur become dormant when collected and stored. In the garden our usual winter weather is all it takes to break the seeds dormancy and trigger germination. Moisture in the soil and frost action both help to break down the hard seed coat so that shoots and roots can grow.
Seeds collected then stored indoors don’t experience frost and some of them need us to provide a cold spell to simulate winter conditions before they will germinate and grow. That’s stratification. Here are a couple of methods you could use.
- Some gardeners prefer to layer the dormant seeds with moist compost in a plastic bag and refrigerate for two months. Everything must be sterile to prevent mould and fungi growing and spoiling the seeds and that is quite difficult to achieve.
- The easiest and quickest way to chill the seeds is to place them in their unopened packet in the freezer for two weeks then sow as normal afterwards. I have always had great success with this method achieving high germination rates and lots of lovely seedlings which grow into healthy strong plants.
Which seeds to stratify?
Most seeds do not need stratification to trigger growth. If you provide a seedbed outdoors in spring or fresh compost and warmth inside with plenty of moisture most seeds will grow quite well even if they are several years old.
However if you come across a batch of seeds that don’t germinate at all then they are probably dormant and need stratification.
The RHS has a booklet entitled Harvested Seed Germination Requirements where you can check if your seeds need any special treatment. I have noticed that Consolida (Larkspur) is not singled out for stratification and yet I just can’t germinate those seeds until they have been in my freezer for two weeks. The photos features in this post are Larkspur flowers I grew this summer (2015) after stratifying the seeds for 2 weeks in the freezer.
The proof is in the pudding as they say here in Lancashire!
Are there any seeds needing special treatment in your garden?