How-to: Grow Perennials

How-to: Grow Perennials

Herbaceous Perennials are plants with fleshy, non-woody stems. The top growth dies back each year, but the root system remains alive and the plant renews growth in spring.

Perennials do not need to be coddled; however, they do need a certain amount of care to keep them disease free and beautifully growing.

The following information will help you select the proper perennials., plant them correctly, and give them the regular maintenance they require.

Soil Prep

When preparing the soil add 20kg of composted manure, one large bale of peat moss and 1 kg of garden food for every 100 square feet. These numbers are general guidelines but you should always follow the manufacturer’s packaging instructions where available. Good drainage is essential. If you have any compost at home feel free to mix this in as well.

When planting make sure all of the roots are well covered and the plant is anchored securely in the soil. Water in well with a transplant fertilizer. After the initial watering, water whenever the soil appears dry. Water early in the day, soaking the soil well and not just sprinkling the foliage. Moisture on the foliage more than 24 hours encourages plant disease.

Cutting Back Perennials

Perennials reappear in spring from buds on their crown or on their roots. They will also grow again and often flower again after being cut down after the main flowering time.

Some important border plants can become unsightly after flowering and should be cut back. Columbine (Aquilegia) or old-fashioned Bleeding Heart (Dicentra spectabilis). The flowering stems of lupins should be removed but leave as many intact leaves as you can. Carolina Lupin (Thermopsis) dies down completely after flowering. Other perennials that go summer dormant are Leopard’s Bane (Doronicum), Virginia Blue Bells (Mertensia) and Oriental Poppy (Papaver orientale). The latter will produce new foliage rosettes in fall. Delphiniums should be cut down to within a hand’s breadth of the ground after flowering to ensure a second flowering in the late summer.

The same severe cutting back applies also to Mountain Cornflower (Centaurea montana), Painted Daisy (Chrysanthemum coccineum also called Pyrethrum), Fleabane (Erigeron), Catmint (Nepeta) and Salvia. Deadheading or the removal of spent flowers will prolong the flowering period of many perennials such as Yarrow (Achillea), Heliopsis, Summer Phlox (Phlox Paniculata) and Scabiosa.

Cutting back Shasta Daisies (Chrysanthemum maximum) will produce stronger and more numerous shoots for next season if cut down early.

Short-Lived Perennials 

The lifespan of a whole range of short-lived or normally biennial plants can be extended by many years if they are cut down after flowering. 

Hollyhock (Alcea), Golden Marguerite (Anthemis), Sweet Rocket (Hesperis), Corncockle (Agrostemma or Lychnis coronaria), Feverfew (Matricaria) and Iceland Poppy (Papaver nudicaule). Alkanet (Anchusa) needs to be cut back right to the root 2cm or more under the ground. 

Some perennials flower themselves to exhaustion, continuing to flower into the fall and subsequently do not produce new buds for the following year. These should be cut back in early autumn to stimulate vegetative growth. This is particularly important in their first year. Coreopsis (Coreopsis grandiflora and lanceolata) but not Coreopsis verticillata, Blanket Flower (Gaillardia), Valerian (Centranthus) and Shasta Daisy (Chrysanthemum maximum). 

Late Flowering Perennial

Tall growing varieties of Asters, Boltonias, Gypsophila and Helenium that may flop or fall apart can be greatly enhanced by being cut back in early summer. This will delay their normal flowering time but much to the benefit of the fall garden. Sedum spectabile (Autumn Joy types) should be cut back by half when the foliage is 30 cm tall.

Woody perennials, more properly sub-shrubs, and others that are not totally herbaceous should be cut back to a hand’s breadth in spring. Basket-of-Gold (Alyssum or Aurinia) after flowering. Rock Rose (Helianthemum) every 2 or 3 years after flowering. Lavender, every 2 to 3 years. Cut flowers annually. Russian Sage, annually in spring. Thyme, every 3 years in spring.

Cutting Back in Fall

Many Ornamental Grasses can be left standing in winter being cut back in spring before new growth begins. Depending much on the attitude of the individual gardener, the stems with the seed heads of Black-eyed Susan, Coneflower, Joe Pye Weed and others can be left to provide food for the birds and to aid in holding snow cover.

The seed heads of Sedum Autumn Joy and the bare stems of the Russian Sage can be things of beauty when encrusted with frost or laced with snow.

Staking and Support

Many perennials will benefit from some type of support to hold the flowers upright where you can enjoy them the most. If you use any materials from your home for this purpose make sure they are of a non-abrasive material so as not to damage the plant stems.

We sell many different type of supports for flowers. Ask one of our staff for further information.


Perennials should be fertilized early in the spring. For best results use a fertilizer formulated specifically for perennials. Our Nutrite Perennial Fertilizer is a proven performer.


A perennial bed will greatly benefit from a layer of mulch at least 3-4 inches thick. Bark, peat moss, compost or seaweed are just a few of the commonly used mulches. Just remember a few things. The mulch should never come indirect contact with the plants. Leave it a few inches away from the stems so as not to cause a rot problem in early spring and late fall. Also, if you use fresh wood shavings these will use up a lot of the nitrogen in the soil which must be replaced!

Insect and Disease Problems

Yes, your perennial bed is just as susceptible to insect and diseases as everything else in your landscape. Healthy, well fed and watered plants will have the best chances of fighting off any problems themselves. But if the problem becomes too severe that’s where you need to stop in and help out. Bring a sample of the infected plant into our nursery and we will gladly diagnose the problem and suggest the appropriate remedy.

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