Growing Primula from Seed
by Claire Cockcroft
(Taken from Primroses Quarterly, Spring 2007 Vol. 65 No. 2, pages 8-10)
Growing primulas from seed is rewarding for several reasons: seed packets are an economical way to produce a large number of plants; seeds are often the only way to acquire some primula species; and finally, seed growing can be quite dramatic. Yes, dramatic! Think of the joy of watching your seedlings come up, the wrath toward pests that tromp through your precious seed pots, the anguish accompanying the dreaded words “damping off”, the triumph of seeing mature plants that you’ve grown from tiny seeds!
I have been growing primula species from seed for the past 15 years. My methods have evolved over time as I learned techniques shared by other growers. Luckily, I started with Primula japonica, a candelabra that is an easy and reliable grower. In general, though, most primulas are relatively easy to germinate if the seed is fresh or has been stored with care. Society seed exchanges are excellent sources of primula species and usually ship seeds to their members in early January. I sow primula seed as soon as it is received to take advantage of the cool, damp weather here in the Pacific Northwest. Any seeds received after the first of March are stored for fall sowing in screw-topped jars in a refrigerator (not freezer) to keep them cool and dry.
The very first step I take in sowing seeds is not to prepare pots or sprinkle seed. It is to record what I am planting plus the seed source and other pertinent details, and to make my labels. I use a computer and spreadsheet for this, as they suit my needs well. In the spreadsheet I include a plant’s cultural requirements as an easy reference for when it comes time to pot up or plant out. In the process, I also gain some hints that may help me grow the primulas on more successfully. I prepare my plant labels using the information from the spreadsheet. As I often sow more than a hundred pots of seed in a year, making plant labels is a tedious task. Nowadays I automate the process by using clear laser address labels on plastic plant markers. Each label has the primula name, source, brief description, some cryptic cultural notes, and the date sown.
Photo: Claire Cockcroft
I use 4”Wx4”D plastic pots; these square pots fit nicely into nursery flats and don’t dry out quickly. An added bonus is that the pots are big enough to accommodate many primulas for a full growing season, allowing me to delay repotting until the young plants are larger and more robust. I use a commercial growing medium. Of the many seed and growing mixes available, look for ones that are loose and quick-draining and avoid ones that are heavily fertilized. If additional fertilizer is needed because you delay repotting, you can easily meet theprimulas’ needs by using a dilute solution of liquid fertilizer during the growing season.
I like to prepare all the seed pots at the same time, filling the pots within a half-inch of the rim and tamping the mix down lightly. Primula seed needs light to germinate, so I top each pot with a thin layer of coarse granite grit before sowing the seed. The grit serves several purposes: it keeps the planting mix damp and in place; I’ve read it can reflect light onto the seeds; and it seems to slightly inhibit the growth of moss. You can presoak the planting medium by placing the prepared pots in a shallow tray and watering them from the bottom. I will admit I don’t always take the time to do this.
Photo: Claire Cockcroft
Over the years I’ve learned the foolishness of my ways in sowing seeds too thickly. Crowded seedlings damp off more readily and are difficult to separate. Nowadays I am careful to scatter the seeds evenly and relatively thinly over the grit. After sowing, I set the pots in a shallow tray of water until the planting mix is saturated. I then
remove the pots to a nursery flat and sprinkle the grit with a light spray of water. Some growers use a dilute fungicide for this sprinkling as a prophylactic against damping off, though I do not.
After watering, I place the nursery flats on a sunny deck and loosely cover them with a row cover such as Remay®. A row cover allows rain to reach the seeds but reduces its force so seeds are not splashed out of their pots. It provides some protection against extremely low temperature and also provides a bit of shade. Lastly, a row cover prevents the birds from eating seeds and pulling up precious seedlings.
Now comes the watching and the waiting. Luckily for us impatient gardeners, many primulas germinate in a few weeks. For example, this year I sowed Primula parryi seeds on January 17 and the first seedlings made their appearance five weeks later on February 21. In another year, PP. alpicola and anisodora sown on January 5 germinated just three weeks later. In most years, about half of the pots show some signs of germination within three months and most but not all will germinate during the first year. A few recalcitrant types take up to 3 years to finally germinate. After that, I’ll toss out the pots and admit defeat.
Photo: Claire Cockcroft
It is important to keep the pots cool and damp but not soggy while you wait. Primulas are able to withstand cold weather fairly well, but will not tolerate drying out and usually will not germinate in hot weather. I bait for slugs and inspect the pots often, both for signs of germination and for cut worms, slugs, and other pests that can ruin a full pot of primulas in a single night. When the weather begins to warm, I move my seed pots to a sand bed on the north side of the house where they remain until I prick them out and pot them on or plant them out.
Both Herb Dickson [APS website] and Angela Bradford [APS Quarterly, Volume 55, No. 1,Winter 1997] mention that you may transplant your seedlings when two to four leaves have developed and both mention that some species, especially the small ones, are best left in their seed pots for a full year to give them time to develop. I will add that I find it easier to divide up a pot full of seedlings when the planting mix is slightly damp, not wet. But that is another article for another issue of the APS