Cold stratification for Sarracenia, Darlingtonia and temperate sundew seed

What stratification does

Some carnivorous plant seeds will not germinate until they have been through a winter. The ICPS seed guide lists a period of damp cold among the special treatments some species need, and notes that for difficult species the timing of sowing can matter as much as the method ^[6]. Stratification is that winter, given either by the real weather or by a refrigerator.

The key word is damp. The ICPS Sarracenia seed page states plainly that seed stored dry in the refrigerator has not been stratified: it has to sit cold and moist, usually among natural materials such as sphagnum ^[1]. Sarracenia seed also has a waxy, water-repellent coat, probably so it can float, and germination cannot start until moisture gets past the wax to the embryo. The same page dismisses quick germination tricks: they produce some seedlings, but good germination needs proper stratification ^[1]. The cold temperate sundew guide sums up what the seed is measuring: time spent damp at 1 to 4 °C, which is ordinary refrigerator temperature ^[3].

How long, and how cold, by plant

The ICPS step-by-step sowing page summarizes the range for the refrigerator method as 4 to 8 weeks depending on species ^[5]. Not every plant from a cold climate needs it: the ICPS filiformis guide notes that seed of the Florida form of D. filiformis germinates without stratification ^[7].

The warm exception: Cephalotus

Cephalotus seed also needs a cool spell, but a mild one. The ICPS guide gives about 15 °C (58 °F) for 6 to 10 weeks, says a refrigerator is too cold, and notes that some seeds take even longer while others can be hurried with the plant hormone GA3. Seeds can be lifted out one by one as they sprout and grown on at room temperature under LEDs ^[9]. Its seed is sown on the surface of a saturated medium such as finely chopped long-fibered sphagnum or fine vermiculite ^[9].

Method 1: a bag in the refrigerator

This is the ICPS method for Sarracenia, and its sowing page uses the same bag for Darlingtonia and the cold temperate sundews and butterworts ^{[1, 5]}.

1. Finely chop a small amount of wet sphagnum moss. Live sphagnum is the best choice for Sarracenia ^{[1, 5]}.
2. Check the moisture. When squeezed, the moss should show water, but there should be no loose water in the bag. If you have to use peat instead, wet it thoroughly and squeeze out most of the water ^[1].
3. Put the moss loosely into a small plastic or snack-size zip bag, add the seed, and spread it evenly through the moss ^[5].
4. Leave plenty of air in the bag, because the seeds need it ^[1].
5. Label the bag and refrigerate it for the period in the table ^[5]. For butterworts, use a few pinches of damp peat (or crumbled vermiculite for P. alpina) and the coldest part of the refrigerator ^{[4, 8]}.
6. When the time is up, spread the moss and seed over the surface of a pot of peat mix with a spoon. For Sarracenia, the ICPS spaces seed about 5 mm apart, spreads it over several pots, and covers it with sand one or two grains deep ^[1]. The Darlingtonia guide also aims for about 5 mm spacing and allows a few millimeters of sand, which can discourage fungus gnats ^[2].
7. Put the pots in plastic bags or under a humidity dome under LED lights, out of direct sun. Sarracenia seed should sprout in 2 to 4 weeks at 20 to 25 °C ^[1].

A variation skips the bag: sow straight into pots and stratify the whole pot inside a plastic bag or under a dome, either in the refrigerator or somewhere that stays around refrigerator temperature and out of the sun, such as a cold garage ^[1]. Darlingtonia seed can likewise be sown directly in the seedling pot, as long as it is stratified one way or the other ^[2].

In an ICPS example, temperate sundew seed spent 4 weeks in chopped sphagnum in the refrigerator, was spread over peat and sand, and germinated in three weeks ^[5].

Method 2: let winter do it

Where the plants are hardy, the weather can do the work.

• Sarracenia. In areas where Sarracenia grow wild, or could, the ICPS suggests sowing outdoors in fall in pots at least 15 cm (6 in) wide of peat and sand, protecting the seed and seedlings from heavy rain until they establish ^[1].
• Cold temperate sundews. The simplest method is to sow in fall and leave the pots outside, and it is the required method for D. anglica. Rain can wash seed from pot to pot, so a cold frame or cold greenhouse helps ^[3].
• D. filiformis. The ICPS calls outdoor winter sowing the best way, because the seedlings come up in step with the seasons and do not go dormant too early ^[7].
• Cold temperate butterworts. Sow outdoors at the start of autumn and move the pots to a bright, sunny spot in spring. The soil must stay very damp, and the seed normally germinates between March and May ^[4]. For P. alpina, the sunny spot also helps protect the thawing seed from mold ^[8].

Keep seedlings in step with the seasons

Seed stratified in the refrigerator still has to fit the calendar. The cold temperate sundew and butterwort guides both advise doing it during your own winter, so the seedlings are in phase with the seasons ^{[3, 4]}. The Darlingtonia guide is more flexible: plants meant to stay in a terrarium can be started at any time, but for outdoor plants it suggests fall if the seedlings will spend their first winter under lights, or late winter if they will go outside in late spring ^[2]. Young Sarracenia can grow under lights for two years, but the second year should follow local sunrise and sunset, because seedlings that are out of sync when moved outdoors can lose a year of growth ^[1]. Winter care for older plants is covered in dormancy.

Seed storage and sourcing

Stratify Sarracenia seed as soon as you can. The ICPS recommends storing packets in a sealed container in a frost-free refrigerator and letting the container reach room temperature before opening it, so that condensation does not wet the seed. Well-stored seed should stay viable for at least a year and possibly two; poorly stored seed may fail within nine months ^[1].

Use seed from cultivated plants or a seed bank, never from wild populations. Several temperate species that need stratification, including Sarracenia oreophila and S. jonesii, are on the ICPS Imperiled Carnivorous Plant Species List, and the ICPS Seed Bank will not accept field-collected seed of listed taxa ^[10]. If you make your own, note that in many areas Sarracenia flowers set no seed unless pollinated by hand, because suitable bumblebees are absent, and that pollinating a clone with its own pollen can lower seed viability and produce more runts ^[1]. Darlingtonia seed is handled much like Sarracenia but must be kept cooler and does not respond well to soil fertilizer ^[11].

What to expect afterward

Patience is built in. Under lights and in heated greenhouses, Sarracenia can bloom about 3½ years after stratification starts; grown the natural way outdoors, it may take two or three years longer ^[1]. Darlingtonia seedlings spend 2 to 4 years in their seedling pot, so the ICPS suggests a mix that lasts, such as 60% washed peat and 40% coarse sand-blasting sand ^[2]. Cold temperate butterwort seedlings can be separated after their first winter, and they need cold winters to survive ^[4]. For the general sowing method, see sowing seeds; to multiply a single favorite plant instead, see dividing Sarracenia.