Fertilizing perennial plants

Horticulture Hints

Gardeners know the importance of providing fertilizer to plants for healthy growth. But a late summer fertilizer application may actually impede the health of your perennial plants.

A good rule of thumb for Iowans is that once the State Fair arrives, it is time to stop fertilizing perennial plants in the landscape. As fertilizer effects may last up to 60 days, late season applications of fertilizer will encourage tender new growth which can be damaged once a killing frost arrives. For woody perennial plants, this can be detrimental to their overall winter survival. As late fertilizer applications encourage tender new plant growth, subject to winterkill damage, time is limited for the new growth to properly harden-off before a winter freeze occurs.

New growth on roses and arborvitae are especially venerable winter damage. Avoid pruning these plants late in the growing season, as the new growth initiated will be subject winterkill from loss of moisture from winter winds and low humidity. Winter moisture loss causes plant desiccation and browning, as once the ground is frozen, plant roots cannot replace moisture loss to the plant.

To prepare your perennial plants for winter, stop fertilization by mid-August and continue to water plants until the ground is frozen. Water trees and landscape plants with at least 1-2 inches of water per week when rainfall is lacking. Apply water slowly, allowing it to absorb into the soil completely around the entire plant. To encourage deeper root growth, it is best to water plants deeply once per week, rather than several times shallowly during the week.

Did you know? During winter sunny days, ground thaw may occur and initiate plant growth, only to be halted once freezing temperatures resume. This freeze-thaw cycle places your perennial plants at risk for winter damage. Applying mulch once the ground has frozen will serve to keep the soil frozen and help prevent your perennial plants from initiating new growth during the winter months.

Questions? Contact McCormick at yvonne@iastate.edu for information or advice.