Cold stratification of seeds
Many seeds are otherwise easy to grow, but need to be cold stratified (usually for between one and four weeks) or they absolutely won't sprout. This means providing a period of cold to mimic winter conditions and usually seeds must be moist for this to be effective, and there are different ways of doing this.
There are also some species which benefit from cold stratification but will germinate without, albeit at lower rates and/or show more sporadic germination. It is likely that in some of these cases, when the seeds are dried, cleaned and stored in cold conditions over winter, a dry cold stratification takes place. We keep our seeds somewhere that is very cold in winter and never really gets warm at other times, but we'd need to do an experiment to determine if that really does makes a difference.
If you obtain seeds while the weather is still cold (before the middle of March) an easy way of growing them is to sow in a seed tray or other container which you can then leave outdoors. Cover the tray with a sheet of glass, propped up at one end to allow air to circulate and water to run off. Cold-stratification will happen naturally and the seeds will begin to sprout when the weather warms up (or they can be brought into a warmer environment after a suitable period in order to force germination).
At other times of the year, seeds can be sown in trays of damp compost and put in a plastic bag in the fridge for a week or more. If space is tight, mix the seeds with a little damp sand, vermiculite, or seed compost in a plastic bag and put that in the fridge. You can also scatter seeds on some damp tissue, fold it up and put it in a plastic bag.
Whatever method you use, it is very important not to over-wet the medium. You want it barely moist, so squeeze tissue or compost well toi remove excess water. Sand should be left to drain for a while after wetting.
Some seeds will do better if they are given a week or so of warmth (usually around room temperature) after bagging up with damp sand or whatever. Some of the more stubborn alliums, such as Allium ursinum, will benefit from this approach.
There are even some really tricky seeds that require double cold stratification as they germinate after two winters in the wild. An example is Yaupon Holly (Ilex vomiteria).
Sow the seeds in compost at room temperature after the stratification period. Seeds will often start to germinate within a week or so of changing temperature and seedlings can be planted into individual pots when they are large enough to handle.
The following species have seeds that absolutely need cold stratification. The pages have more planting details and specific information on the length of the cold period required for each.
Alliums
Scots lovage (Lingusticum scoticum)
Caucasian spinach (Hablitzia tamnoides)
Virginia skullcap (Scutelaria lateriflora)
Yaupon (Ilex vomitoria)
The following species have seeds that will benefit from cold stratification, but will also give fair germination rates without (or possibly with 'dry cold stratification').
Good King Henry (Chenopodium bonus-henricus)
Valerian (Valerian officinalis)
Vervain (Verbena officinalis)
There are also some species which benefit from cold stratification but will germinate without, albeit at lower rates and/or show more sporadic germination. It is likely that in some of these cases, when the seeds are dried, cleaned and stored in cold conditions over winter, a dry cold stratification takes place. We keep our seeds somewhere that is very cold in winter and never really gets warm at other times, but we'd need to do an experiment to determine if that really does makes a difference.
If you obtain seeds while the weather is still cold (before the middle of March) an easy way of growing them is to sow in a seed tray or other container which you can then leave outdoors. Cover the tray with a sheet of glass, propped up at one end to allow air to circulate and water to run off. Cold-stratification will happen naturally and the seeds will begin to sprout when the weather warms up (or they can be brought into a warmer environment after a suitable period in order to force germination).
At other times of the year, seeds can be sown in trays of damp compost and put in a plastic bag in the fridge for a week or more. If space is tight, mix the seeds with a little damp sand, vermiculite, or seed compost in a plastic bag and put that in the fridge. You can also scatter seeds on some damp tissue, fold it up and put it in a plastic bag.
Whatever method you use, it is very important not to over-wet the medium. You want it barely moist, so squeeze tissue or compost well toi remove excess water. Sand should be left to drain for a while after wetting.
Some seeds will do better if they are given a week or so of warmth (usually around room temperature) after bagging up with damp sand or whatever. Some of the more stubborn alliums, such as Allium ursinum, will benefit from this approach.
There are even some really tricky seeds that require double cold stratification as they germinate after two winters in the wild. An example is Yaupon Holly (Ilex vomiteria).
Sow the seeds in compost at room temperature after the stratification period. Seeds will often start to germinate within a week or so of changing temperature and seedlings can be planted into individual pots when they are large enough to handle.
The following species have seeds that absolutely need cold stratification. The pages have more planting details and specific information on the length of the cold period required for each.
Alliums
Scots lovage (Lingusticum scoticum)
Caucasian spinach (Hablitzia tamnoides)
Virginia skullcap (Scutelaria lateriflora)
Yaupon (Ilex vomitoria)
The following species have seeds that will benefit from cold stratification, but will also give fair germination rates without (or possibly with 'dry cold stratification').
Good King Henry (Chenopodium bonus-henricus)
Valerian (Valerian officinalis)
Vervain (Verbena officinalis)
