Gadenin日本語Get the appCool weather makes the flowers, warmth ends them
Primulas set and open their buds in cool conditions. Flowering is strongest between 40F and 60F (4C and 16C), slows above 65F (18C) and shuts down almost entirely above 75F (24C). This single fact explains most of what people find puzzling about them: why a plant flowers for two months outdoors and three weeks on a windowsill, why the display collapses the week the weather turns warm, and why a plant in shade is still going when the one in sun has finished.
It also means the way to a longer display is not feeding or watering harder but moving the plant somewhere cooler. A potted primula kept at 55F to 60F (13C to 16C) in a bright, unheated room, a porch or a sheltered spot outdoors will hold its flowers two to three times as long as one on a warm windowsill above a radiator. Outdoors the same rule applies: the coolest, most shaded corner is the one that flowers longest.
Flowers that never open, or open on stalks that are too short
Buds that form, sit tight and then go brown without opening are almost always a moisture problem, either dryness at the root while the buds were forming or a spell of cold wet weather that let gray mold into the packed bud cluster. Check the compost a finger deep first, then look into the middle of the plant for a fuzzy gray coating on the failed buds and pick every affected one out.
Flowers that open crowded down among the leaves on very short stalks are usually behaving normally for the type; several primulas carry their flowers in a tight cushion at leaf level by design and are not trying to make a tall stem. Where short stalks are a genuine fault is when the same plant flowered on longer stalks last year, and then the cause is heat during bud formation or a plant that has become congested and is competing with itself.
A plant that makes plenty of healthy leaf and no buds at all has usually been fed too generously, or is in too much shade rather than too little. Deep shade under evergreens, where the light stays dim even in early spring, gives you leaves. Move it to somewhere that is bright and open while the trees are bare, and shaded only once they come into leaf.
| What you see | Most likely cause | What to do |
|---|---|---|
| Display ends abruptly after a warm week | Temperature above 75F (24C) | Move to a cooler shaded position; nothing else will extend it |
| Buds brown and dry without opening | Root ball dried out while budding | Soak thoroughly, then keep evenly damp |
| Buds gray and furry, collapsing | Gray mold in cold wet conditions | Remove every affected bud; improve air movement |
| Plenty of leaf, almost no flower | Too much nitrogen, or too deep shade | Stop feeding; move to spring sun and summer shade |
| Smaller flowers each year | Congested clump | Lift and divide as soon as flowering finishes |
| Whole plant collapses in flower | Roots eaten, or crown rot | Lift and inspect the roots before doing anything else |
Deadheading, and a second flush in autumn
Take spent flowers off by following the stalk right down to the base of the plant and pulling or cutting it there. Leaving the stub means the plant carries on ripening seed, and a plant in seed stops making buds. Done regularly through the flowering period, deadheading commonly adds two to three weeks to the display and keeps the plant looking clean rather than brown at the center.
Many primulas will flower again in the cool of autumn if the summer has not exhausted them, sometimes lightly and sometimes generously. That second flush is a reward for a plant that has been kept damp and shaded through the hot months, and it is a good sign that the clump is healthy going into winter. There is no need to remove those late flowers; they will be finished before hard frost.
Leave a few seed heads deliberately if you want self sown seedlings. They germinate in the damp shade around the parent, appear as small rosettes the following year and flower in the year after that. It costs nothing and steadily fills a shady border.
Note the date of the first open flower and the date the last one fades, for each position you grow them in. Two seasons of those pairs tell you exactly which parts of your garden stay cool longest, and that is where the primulas should go.
What to do the moment flowering finishes
The end of flowering is the busiest point in the primula year, and it is a narrow window. The plant is still in full growth, the soil is still damp and cool, and everything you want to do to it should be done now rather than left until autumn.
Three jobs, in order: clear off all the old flower stalks; plant out any pot grown plants that were bought in flower, into shade at the same depth they were growing; and lift and split any clump that has been in place two or three years, replanting single crowns a hand's width apart. Then mulch around them all, keeping the mulch clear of the crowns, and water.
After that they need very little until autumn beyond not being allowed to dry out. Resist tidying away the leaves in summer even when they look tired. Those leaves are what builds next spring's flower buds, and a plant stripped of them in midsummer flowers poorly the following year.