Fertilize Herbaceous Perennials Wisely, Lightly

Linda Benedict  |  8/25/2008 11:15:32 PM

Herbaceous perennials such as the purple coneflower are increasingly popular in landscapes. (Photo by Yan Chen)

Table of formula

Yan Chen and Regina P. Bracy

Herbaceous perennials are winter hardy ornamental plants that reappear each spring from their crowns or root systems. Many species can be used as groundcovers or landscape plantings to provide color for extended seasons. Some major species in production and landscape use are daylily, lantana, verbena, purple coneflower and black-eyed Susan.

When good cultural practices are followed, herbaceous perennials can establish themselves quickly and create color in the landscape. Once established, they have relatively low requirements for maintenance. However, nutrient requirements during their establishment are unknown for many species in this group. Because fertilizer is the least expensive input for maximum plant growth, landscape contractors and homeowners tend to achieve fast establishment by over applying fertilizer at planting. This is inefficient and often results in increased environmental risks such as water contamination by nitrate leaching and runoff from excess fertilizer.

Over the past two years, researchers at LSU AgCenter’s Hammond Research Station have conducted field studies to determine the effects of nitrogen fertilizer by type (tablet versus granular), application rate (0 to 4 pounds per 1,000 square feet) and application timing (single or split) on the establishment of seven herbaceous perennials in a full-sun landscape.

The experiment was conducted in a landscape research area that had not been fertilized for several years. Soil in this area is a sandy loam with one percent organic matter. About 4 inches of pine bark were incorporated into the top 6 inches of soil to make raised plots with a 2-foot alley between plots to avoid roots intruding into other plots. Seven perennial species: cigar plant, daylily Stella d’Oro, guara Siskiyou Pink, Mexican heather, lantana New Gold, purple coneflower and black-eyed Susan Goldstrum were planted and treated with different fertilizer regimes.

Two types of fertilizer – OsmocotePlus 16-8-12 tablet (0.26 ounces, 3 - 4 months) and OsmocotePlus 15-9-12 granular (5 - 6 months) – were studied in combination with different application methods (Table 1). At planting, granular treatments were applied broadcast and incorporated into the top 4 inches of soil; tablet treatments were applied by dropping the tablet next to the plant root ball.

Plant response to increasing fertilizer rates: When plants were not fertilized, nitrogen deficiency and poor growth were observed in all species except daylily. When fertilized, all species, except daylily, responded positively to increasing fertilizer rates, and four pounds of nitrogen per 1,000 square feet produced in largest plants. However, because large-size plants often lack compactness and require more pruning, they received quality ratings lower than plants that were in lush growth but also were compact and had better flower display.

Comparison of fertilizer types: Better plant quality was observed in cigar plant, lantana and Mexican heather when two tablets per plant or two pounds of nitrogen per 1,000 square feet were used. These two treatments also resulted in better flower display in purple coneflower and black-eyed Susan. The high fertilizer rate – four pounds of nitrogen per 1,000 square feet – produced more flowers, but plant quality ratings were lower because of their large size and lack of compactness. As a result, applying two tablets per plant or two pounds of nitrogen per 1,000 square feet resulted in optimum quality of most plants in May (Table 1). When compared with two pounds of nitrogen per 1,000 square feet, two tablets per plant provided similar season-long nutrients with reduced total fertilizer use.

Overwinter survival rate: Daylily and lantana plants overwintered well regardless of fertilizer regimes. These plants need only minimum fertilizer to establish. Two tablets per plant resulted in higher survival rate (75 percent) in black-eyed Susan, cigar plant and Mexican heather. A single application of two tablets in guara produced results similar to a split application of one tablet at planting plus one pound of nitrogen per 1,000 square feet in July.

Based on these results, we recommend two tablets (0.26 ounce each) per plant at planting to establish most medium- to large-size herbaceous perennials such as lantana, black-eyed Susan, Mexican heather and purple coneflower. The LSU AgCenter recommends a low fertilizer rate at one pound of nitrogen per 1,000 square feet for daylilies.
Yan Chen, Assistant Professor, and Regina P. Bracy, Professor, Hammond Research Station, Hammond, La.

(This article was published in the summer 2008 issue of Louisiana Agriculture.)

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