Many seeds are simple enough to start. Just add moisture and maybe a little heat and they’re on their way. But there are also plenty of difficult seeds that require special treatment before they’ll put down roots.
For instance, some seeds need specific light conditions to germinate. Some require a few months of cold weather. Others may need a good soaking or light sanding to soften up their tough exteriors.
And still others may need a couple of these treatments in combination before they’re “convinced” that it’s finally safe to sprout.
Light or Dark?
Ornamentals such as Chinese lanterns, snapdragons, Oriental poppies and begonias require light to germinate. So do yarrow, red-flowering salvia, tickseed coreopsis and dill, among others.
And, as for one of my favorite novelties, the “eyeball plant”? (It’s also known as “toothache plant” or Spilanthes.) I accidentally discovered that its seeds need light, too.
After carefully planting, watering and then covering them completely, I spilled several extra of these difficult seeds on top of my growing medium. While the randomly scattered seeds took root, their in-the-dark counterparts never germinated.
Conversely, there are many varieties which won’t germinate in light. Some denizens of the dark include the sensitive plant—another favorite novelty plant—as well as borage, fennel, sweet peas, calendula and nasturtiums. Bachelors buttons, delphiniums, pansies and violets also need darkness to sprout.
Some seeds—think morning glories, nasturtiums, lupines, dried peas and okra—have extremely tough, outer seed coats. Unless moisture can penetrate these, germination will be a no-go for these difficult seeds.
You can break through tough seed coats by lightly sanding them. Just don’t overdo it. When I began growing luffa gourds, I sanded my seeds a little too vigorously. And, although the luffas had sprouted in a hurry, their first leaves were damaged, ultimately setting each plant’s progress back.
Want a gentler way to treat hard seeds? Try soaking them in hot water for 12 to 24 hours before planting.
Individual seeds should soften and swell as they take on water. When I notice this has happened, I remove and immediately plant each of those slightly swollen seeds.
For any remaining seeds that haven’t changed in appearance? You can pour off their water, replace it with new hot water, and allow them to soak for a little longer. Just watch out for signs of rot.
Into the Cold
Perhaps the most common reason seeds fail to germinate? Many seeds—particularly those of native perennial plants and trees—require a lengthy period of cold, moist weather to break their internal dormancy.
Milkweed, purple coneflower, black-eyed Susans, lavender, columbine, violets and pansies are just some of the common seed types that need a brisk cold snap to get going.
To simulate a period of winter, you can cold stratify these seeds in your refrigerator. A period of four to six weeks usually will suffice, although some plant varieties may require lengthier stays.
If you can spare the space, the simplest way to cold stratify your seeds is to go ahead and plant them in containers of growing medium, water and allow to drain, then place the containers in a plastic bag.
Don’t seal the bag completely—seeds need exposure to a combination of cold temperatures, air and moisture during this process. Label the containers or the bag with the seed variety and the date you began cold stratification.
If space in the fridge is at a premium, you can skip the container and simply mix your seeds with moist, sterile growing medium inside a re-sealable plastic bag. Again, be sure that air is able to circulate by leaving a portion of the seal open or by poking some ventilation holes in the bag.
Periodically check to make sure the growing medium is moist.
Keep ‘Em Cool
When it comes to starting chamomile, rosemary and thyme, don’t bother with a seedling heat mat.
These herbs require a growing medium that’s about 55 degrees F. Likewise for coral bells, sweet peas, and poppies.
Because poppies don’t transplant well, your best bet is direct sowing these when your outdoor temperatures are right. Outdoor planting in late spring rather than starting seeds indoors is a great way to go for some other cold-tolerant, transplant-resistant types like borage, parsley, and dill.
Finally, for best results with seeds requiring special treatment, consider purchasing new stock with clearly stated germination rates.
Of course, if you have enough lead time, you can perform your own germination testing. Provided your seeds are viable—and you follow the right steps—you should be able to start even the most finicky varieties.