What's Blooming: Seed Stratification

 

What’s Blooming:  Seed Stratification

by Sherry Fuller, Curator, Gardens on Spring Creek 

In January, the majority of blooms at The Gardens are indoors.  But a lot is still happening behind the scenes (and under the ground) in preparation for the beauty, color and fragrance of blooming times ahead. 

Most varieties of annuals and vegetables are very easy to germinate and grow, but some kinds of perennials have seeds with built-in dormancies that prevent them from germinating with just warmth and water. 

These dormancies are an inherent protection that prevents the seeds from growing right when they are dropped from the mother plant.  This is often done in autumn – not an ideal time to begin growing.  Their dormancy period allows them to hold off on germination until the spring when they have the full year ahead to grow. 

Most often, this dormancy is broken by a wet / cold cycle – winter in the natural world – called stratification.  When starting perennials from seed in a cultivation setting, it often takes a little nudging.

Those familiar with starting perennials from seed may know to store seeds in the refrigerator - but that isn’t always effective.  Most stratification strategies involve mixing seeds with a little damp sand or potting soil and storing this in sealed baggies in the fridge or freezer for four to eight weeks.  After that time, the mix can be spread over the surface of pots and will hopefully grow.  I’ve tried this technique numerous times over the years with very mixed results. 

Of course, simply planting seeds outside in the fall will naturally stratify seeds, giving them the freeze / thaw cycle they need.  But since perennials can germinate and grow into mature plants quite slowly, they sometimes get lost in the spring’s weeds or we simply forget to water them during their critical first weeks after germination.  

A more reliable way to stratify seeds involves spreading them over three- or four-inch pots of potting soil, then covering the seeds with fine gravel or squeegee (an unscreened gravel mix that results in grains of multiple sizes).  They then need to be watered well, placed outside in a shady location and kept covered as much as possible with snow.  If the weather doesn’t cooperate, remember to water your pots every couple of weeks.  At the Gardens on Spring Creek, where we deal with rabbits, we cover the trays of pots with chicken wire to keep them safe. 

 With spring’s warmth, usually in late April or May, the seedlings will begin to appear.  At that time, we move all our trays to a sunny location and water them more regularly.  Later in the summer, seedlings are large enough to transplant, so our team gently pulls apart the little plants and moves into their own small pots. 

 

Another type of dormancy is often encountered in legumes and a few other hard seeds.  These need scarification, a fancy term for damaging the seed coat enough to allow water to penetrate and the seedling to emerge.  Seeds can be rubbed between sheets of sandpaper or in sand.  Larger seeds can be nicked with nail clippers or a file.  Sometimes just soaking the seeds in very hot water will be enough to soften the seed coat. 

Deciding which seeds to treat and which to simply plant in spring can be tricky.  I’ve found lists of plants that need stratification to be very misleading – probably because seed ripeness, age and other factors that play into their viability.  Over the years, I’ve often just used trial and error to decide seed treatments, but if you’ve only got a few seeds and really want them to grow into plants quickly, this isn’t the best option.  

I find Jelitto Perennial Seeds and Western Native Seeds to be good, reliable sources of information on individual species’ needs.  You might also find that some perennial varieties will germinate a bit with no treatment, but percentages will improve with a treatment.

Another important lesson I’ve learned is to keep your seeds labeled at all times.  While you might be able to identify the seed heads you collected in the fall, cleaned seeds and tiny seedlings all look much alike.  

Sherry’s Lists for Seed Treatment:

Statification: 

·       Baptisia

·       Clematis

·       Delphinium

·       Gentians

·       Most Penstemon

·       Pulsatilla

·       Sphaeralcea

 

Scarification / Soaking in Hot Water:

·       Lupine

·       Morning Glory,

·       Perennial Sweet Pea

 

Seeds Not Requiring Any Treatment: 

·       Achillea

·       Campanula

·       Columbines

·       Dianthus

·       Most Echinacea

·       Most Evening Primrose

·       Flax

·       Gaillardia

·       Most Grasses

·       Hollyhocks

·       Ratibida