Polyanthus Plant Care

Being led down the primrose path isn't a pleasant experience, unless that path weaves between borders of polyanthus primroses (Primula x polyanthus). When bright, beguiling five-petaled flowers nod above the primroses' broad green leaves, spring is on the way. Happiest in cool, woodland settings, they're marketed as perennials in U.S. Department of Agriculture plant hardiness zones 3 through 8. In reality, only loving care brings polyanthus primroses through a hot summer. If they receive it, don't be surprised by a repeat flush of flowers in fall.

Polyanthus primroses need very little supplemental fertilizer. Excessive fertilizer buildup can harm them. Fertilize the primroses with water-soluble tomato food diluted to one-half the label's recommended strength when their buds start showing in spring.

The maker of one 17-18-28 tomato food formula recommends dissolving 1 teaspoon of the granules in 1 gallon of water, so for that brand you'd reduce the food to 1/2 teaspoon per 1 gallon.

Fertilizers come in different strengths, so always follow the label's directions concerning the amount and frequency of application.

Fertilize the primroses every 10 days until their blooms start opening, and once more when the last flowers fade.

These primroses need consistently moist soil. They're happiest where summers are cool and rain is plentiful. Otherwise, their best chance of surviving heat or drought is a post-bloom move from their sunny spring location to a partially shady one.

Select a site with humus-rich soil, or amend one with one or more of the following:

Organic compost

Rotted leaves

Well-aged manure 

Loosen the top 9 inches of soil with a spade or rototiller. For sandy or clay soil, work a 4- to 5-inch layer of the amendments into the loosened soil; if it's loam, a 2- to 3-inch layer is enough.

Water the primroses well and set them in their new location at the same depth they were in the original one. Tamp the soil lightly to remove air pockets, and water again.

Overwinter the primroses in their summer home, and return them to a sunny spot in early spring.

If the soil in the summer location alternately freezes and thaws during the winter, cover it loosely with straw or leaves.

As woodland plants, primroses benefit from a 2- to 3-inch layer of mulch of chopped leaves. They also need 1 inch of rain or supplemental water each week. An inch of rain amounts to about 6 gallons of water for each 10 square feet of soil. Water plants in loam or clay soil once a week. For sandy soil, water twice weekly with 3 gallons per 10 square feet.

Slow, deep watering around the bases of the plants is much better than overhead sprinkling.

 An inexpensive rain gauge keeps track of the weekly rainfall. If it holds 1/2 inch of water at the end of the week, for example, give the primroses 3 gallons of water.

Empty the gauge after taking the measurement.

When the primroses' flowers fade, cut their entire stems off at the base. As the older leaves start to yellow and decay, prune them as well. Regular grooming keeps the plants tidy and encourages new flowers and foliage. Groom the plants with clean, sharp stem cutters disinfected in rubbing alcohol between cuts to keep disease form spreading.

Diseases don't bother polyanthus primroses, but pests are a different story. Sap-sucking aphids, whiteflies and mealybugs plague them, along with leaf-chewing spider mites. The insects cover the plants in sticky, clear waste, and the mites spin fine webs on the backs of the leaves.

To manage the pests, prune the infected leaves or spray the plants with organic, ready-to-use insecticidal soap until all their surfaces -- including the backs of the leaves -- drip. Repeat the soap application every two to three days until the infestation subsides.

To avoid leaf burn, spray when no sun is hitting the plants.

Wear protective eyewear, waterproof gloves, a long-sleeved shirt and long pants, and follow the label directions when using the soap.