Sweet Clover Seed Production
Introduction
Sweetclover is generally biennial, with two main types:
- White Sweetclover (Melilotus alba): Recommended Manitoba variety: Polara (low coumarin).
- Yellow Sweetclover (Melilotus officinalis): Recommended varieties: Norgold (low coumarin) and Yukon (“Madrid” type). Yellow types are earlier, finer-stemmed, less productive for forage but more reliable for seed.
Growth Characteristics
- Year 1: Single branched stem and taproot; crown buds and food reserves develop late season.
- Year 2: Crown buds produce stems using stored reserves; no replenishment occurs.
Storage of food reserves in roots and crown buds depends on photosynthesis in the leaves of the single stem. Any reduction in top growth reduces food storage. Thick stands or companion crops may limit viable axillary buds, reducing regrowth potential.
- Management Note: Avoid clipping the main stem too low in year one to preserve axillary buds for regrowth.
Site Selection
- Adapted to a wide range of soils; moderately drought-tolerant and winter-hardy.
- Best on fertile, well-drained clay or clay loam soils; tolerates saline soils.
- Avoid acidic or flood-prone soils.
- For pedigreed seed, follow Canadian Seed Growers’ Association for isolation requirements.
Site Preparation
- Minimize weed competition through cultural practices before seeding.
- Sweetclover is sensitive to most broadleaf herbicides.
Seeding
- Seed Treatment: Scarify seed, remove the hull and scratch the hard coating (usually done by seed suppliers) and inoculate with Rhizobium melilotii.
- Seeding Rate: ~6 lb/ac (7 kg/ha) for close-drilled stands; aim for 1–2 plants per sq. ft (19–20/m²).
- Depth: <1 inch (2.5 cm).
- Timing: Seed early in spring for best moisture conditions.
- Companion Crops: Optional; if used, seed companion crop first at ½–⅔ normal rate, then cross-seed sweetclover.
- For pedigreed seed, use spaced rows for easier roguing.
Fertilization
- Nitrogen usually supplied by fixation if inoculated properly.
- Apply phosphorus, potassium, or sulfur as indicated by soil tests.
Pollination
- Honey bees are essential; 1–2 colonies per acre (3–5/ha) recommended.
- Leafcutter bees and other pollinators also contribute.
Harvesting
- Sweetclover produces abundant seed but is prone to shattering.
- Best method: Windrow when 50–60% of pods are brown, black, or white.
- Cut when plants are damp (dew or rain) to reduce shatter.
- Cure for several days to a week, then thresh using slow cylinder speed and wide concave clearance.
- Avoid direct combining—shattering losses are severe at advanced maturity.
Diseases
- Common issues: damping-off, root/crown rot, stem and leaf diseases.
- Typically light incidence; slightly higher on seed stands but rarely serious.
Insect Pests
- Sweetclover weevil: Main pest; damages seedlings and second-year stands.
- Control: insecticides, tillage after harvest, and field rotation.
- Cutworms and grasshoppers: May damage seedlings; control with cultural practices and/or insecticides.
- Insects can be controlled by cultural methods and/or use of insecticides.
