Why didn't my Cleome come up? Seed germination and light

Stages in the germination of Moonflower seeds

When I was a new gardener I could not understand why my cleome didn’t germinate. Everyone else always had fields of it, an exaggeration, but it seemed as if they did compared to my bare patch. As I found out, cleome needs to be sown at the last frost date, when nights are still cool and days warm up nicely, for they are half-hardy annuals. And they should never be covered, just pressed into the soil, as they need light to germinate.

Different seeds need to be sown at different depths. A traditional approach to sowing depth is to plant a seed at a depth that is three times its thickness. So, a larger seed, such as a bean, would be planted about one or two inches deep, whereas tiny seeds, such as those of poppies or tobacco, should barely be covered, if at all. If disturbance is a concern, particularly when watering, press in and cover these types of seeds with a very thin layer of horticultural vermiculite, which will allow light to filter through.

The seed embryo is a living breathing little plant to be, one that lies dormant until light, moisture, temperature and air in correct amounts are detected. Then germinating hormones are triggered and it begins to grow.

Moonflower seedling
It has been suggested that light is required by some tiny seeds because it is a signal that they are close to the soil surface, and will therefore be able to sprout easily, as opposed to larger, more robust seeds, which can more easily push their way through the soil to the surface upon sprouting.

Surface sow these as they germinate better when exposed to light:

  • Agastache
  • Ageratum 
  • Amaranth
  • Ammi
  • Balloon flower
  • Bells of Ireland
  • Blue Woodruff
  • Chocolate Daisy
  • Cleome 
  • Columbine
  • Coreopsis
  • Corkscrew Vine - try this if germ difficult
  • Feverfew
  • Foxglove
  • Globe Thistle 
  • Hollyhock
  • Lobelia 
  • Maltese Cross 
  • Mexican Hat 
  • Mignonette
  • Nicotiana
  • Petunia
  • Poppy
  • Primula
  • Purple Berkheya
  • Rose Campion
  • Rudbeckia 
  • Salvia
  • Snapdragon 
  • Stock-Evening Scented 
  • Swamp Milkweed
  • Sweet Alyssum
Cleome seedlings in a seed flat

Most seeds are not fussy and will germinate in either dark or light, a few require darkness to germinate and light actually inhibits germination.

Make sure you cover these seeds as they need darkness to germinate:

  • Calendula 
  • Cornflowers
  • Gomphrena 
  • Larkspur
  • Nasturtium
  • Painted tongue
  • Phlox
  • Primula (most)
  • Verbena
  • Viola
  • Zinnia


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