Nutrient Requirements of Herbaceous Perennials in Newly Established Landscape Plantings

Herbaceous perennials are a group of plants with increasing popularity in landscapes. Photo shows Echinacea purpurea (coneflower)

Herbaceous perennials are continuously gaining popularity in landscape plantings. The wholesale value of this group has been increasing 5 to 8 percent annually since 2003 with a total crop value of $712 million in 2006 (NASS, 2007). The business goal of lawn- and garden-care companies emphasizes aesthetic value of the urban landscape. Improper nutrient management, such as overapplication of fertilizer, is inefficient and may result in increased pest problems and risks of contaminating ground and surface waters by nutrient leaching and runoff (2). However, nutrient requirements are still unknown for many species in the herbaceous perennial group during their first year establishment in the landscape. 

Fertilization recommendations for herbaceous perennials vary dramatically from source to source, and nutrient requirements among species can be very different (3). Generally, two applications of controlled release fertilizer (CRF), one incorporated at transplant and one topdressed in the middle of a growing season, are recommended for herbaceous perennials. Tablet-type CRF may provide season-long nutrients and reduce total amount of fertilizer used per unit area. The objective of this study was to determine effects of fertilizer type (tablet vs. granular), application rate (0 to 20 g N•m^-2) and application method (single or split application) on the first-year growth and flowering of seven flowering herbaceous perennials in full-sun landscapes.

The experiment was conducted in a newly established landscape research area that had not been fertilized for several years. Soil in this area is a sandy loam with 1% organic matters, 38 mg•l^-1 P, 53 mg•l^-1 K, 499 mg•l^-1 Ca, and 123 mg•l^-1 Mg with soil pH at 5.3 (Soil Testing and Plant Analysis Laboratory, LSU). About 2 inches of pine bark were incorporated into the top 6 inches soil, and dolomitic limestone was incorporated at 70-lb/1000 ft² to adjust pH. Raised plots (10’ x 12’) were made with a 2-foot alleyway in between every two plots to avoid plant roots and fertilizer intruding into other plots. Experimental design was a split-plot design. Whole plots were arranged as a completely randomized block design with seven fertilizer treatments and 4 blocks. 

Seven perennial species -- cigar plant (Cuphea ignea), daylily (Hemerocallis spp.) ‘Stella d’ Oro’, Guara (Guara linfheimeri ) ‘Siskiyou Pink’, Mexican heather (Cuphea hyssopifola), lantana (L. camara) ‘New Gold’, Purple coneflower (Echinacea purpurea) and blackeyed Susan (Rudbeckia hirta) ‘Goldstrum’ -- were randomly arranged in each fertilizer treatment as sub-plots. Two plants of each species (3 plants of daylily) were planted next to each other as sub-samples.

Fertilizer types: OsmocotePlus 16-8-12 tablet (7.5 g, 3 - 4 month) or OsmocotePlus 15-9-12 (5 - 6 month) and application methods: single application at transplant (April 22, 2005) or split application with one applied at transplant plus a topdressing on July 20 were studied. Treatment combinations were: 1) unfertilized, 2) two tablets per plant (= 1.32 lb N/1000 ft² at 2 feet plant spacing), 3) OsmocotePlus 15-9-12 at 1 lb N/1000 ft², 4) OsmocotePlus 15-9-12 at 2 lb N/1000 ft², 5) split application of treatment 4 with half incorporated at transplant and half as topdressing in July, 6) OsmocotePlus 15-9-12 at 4 lb N/1000 ft²; 7) one tablet per plant (= 0.66 lb N/1000 ft² at 2 feet plant spacing) at transplant and topdressing of OsmocotePlus 15-9-12 at 1 lb N/1000 ft² in July. 

At transplant, granular CRF treatments were incorporated into the top 4” of soil, and tablets were dropped next to the root ball of a plant. All plots were mulched with pine straw (4 inches). Topdressing in July was applied as broadcast to the whole plot. Plots were irrigated by overhead micro sprinklers controlled by a timer to supply about 1½ inches of water per week before May 15 and 2½” inches of water per week thereafter. Irrigation was automatically postponed at ½” rainfall events by a rain sensor. 

Nitrogen deficiency symptoms, including lower leaf chlorosis and poor plant growth, were observed in all species except daylily when plants w

Hammond Research Station staff members taking plant growth measurements in the field

ere unfertilized. No nutrient deficiency was observed in fertilized plants regardless of fertilizer regimes. Tissue N content in non-deficient plants of C. ignea, C. hyssopifola, guara, lantana, E. purpurea and rudbeckia were in the range of 2.6–2.8%, 1.6–2%, 2.1–2.8%, 2.1–2.6%, 2.9–3.2%, 2.4–3.0%, respectively. These ranges represent the sub-optimum nutrient status (no deficiency symptom yet) at the low rate (1 lb N/1000 ft²) and over-consumption of nutrients at the high rate (4 lb N/1000 ft²) when these species were fertilized. Tissue N content in unfertilized daylily plants was 1.8% and was in the range of 1.9 to 2.4 in fertilized plants. Compared with unfertilized plants, all species except daylily responded positively to fertilization, with higher growth index (GI) under increasing fertilizer rates and 4 lb N/1000 ft² resulted in the highest GI in August. 

Two tablets per plant and the 2 lb N/1000 ft² resulted in GI that was usually higher than the rest of treatments in C. ignea, lantana and C. hyssopifola (only data of C. ignea are shown in Figure 1). The lack of response in daylily was similar to the lack of response found in iris (3). Total number of flowers per plant from April to June suggests that when fertilized with 4 lb N/1000 ft² or 1 or 2 tablets CRF per plant, daylilies had higher number of flowers than unfertilized (50, 49, 48 vs. 31). The 2 lb N/1000 ft² or the split application of the 2 lb N/1000 ft² or the 1 tablet plus 1 lb N/1000 ft² resulted in significantly fewer flowers than the above three treatments but still more than unfertilized. 

Total number of flowers in R. hirta and E. purpurea from May to August were significantly different under different fertilization regimes with 4 lb N/1000ft² resulting in the highest numbers of flowers. Two tablets per plant and 2 lb N/1000 ft² resulted in acceptable flower display in E. purpurea. Two tablets per plants and 1 or 2 lb N/1000 ft² resulted in acceptable flower display in R. hirta. Overall quality ratings of plants in the 4-lb. N treatment, however, were not significantly different from those of plants in some other treatments because large plant size led to less compactness and flower performance. Two tablets per plant and the OsmocotePlus 2 lb N/1000 ft² resulted in optimum quality of many species in May. Split application of tablet and CRF at a low rate resulted in good flower display in daylily but lower plant quality than a single application of two tablets at transplant.&nbs

Plant growth index of C. ignea fertilized with two types of CRFs at different rates from May to August, 2005. Treatment 1 to 7 correspond to the treatments described in the materials and methods section.

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Survival rate was significantly different among species. Daylily and lantana had 100% survival. Two tablets CRF per plant resulted in higher survival rates in R. hirta, C. ignea, and C. hyssopifola. Four lb. N/1000 ft² resulted in similar results in Cuphea plants but not R. hirta. Split applications resulted in similar results as single application of two tablets in guara but lowered survival rates in other species.

Based on these results, we recommend two tablets per plant or 2 lb. N/1000 ft² for large-size herbaceous perennials such as C. ignea (cigar plant), lantana and C. hyssopifola (Mexican heather), R. hirta (rudbeckia) and E. purpurea (coneflower). We recommend a low fertilizer rate for daylily at 1 lb. N/1000 ft²R.

References

1. Beverly, R. B. et al. 1997. Fertilizer management by landscape maintenance and lawn care firms in Atlanta. HortTechnology 7(4) 442-445.

2. Proctor, C.L., S.L. Warren, D. J. Werner, and R.L. Mikkelsen. 2003. Influence of N rate and timing on nitrate leaching in a simulated landscape of herbaceous perennials. SNA Res. Conf. 48:387-390.

3. Proctor, C.L., S.L. Warren, R.L. Mikkelsen, D.J. Werner and H.T. Kraus. 2004. Effect of nitrogen rate and nitrogen timing on herbaceous perennials in a simulated landscape. SNA research conference vol. 49. 441-445.