Wisconsin’s late spring frosts mean nasturtium seeds should go into the ground only after the soil has warmed and all danger of cold nights is gone. Knowing the best time to plant nasturtium in wisconsin is key to getting those bright blooms and edible leaves before the first fall frost hits. If you plant too early, the seeds rot in cold, wet soil. If you plant too late, you miss weeks of flowers.
Best Time To Plant Nasturtium In Wisconsin
For most of Wisconsin, the sweet spot for planting nasturtium seeds outdoors is between mid-May and early June. This timing lines up with the average last frost date across the state. In southern Wisconsin, that date is around May 10-15. In northern Wisconsin, it can be as late as May 25-30. Always check your local frost forecast before putting seeds in the ground.
Why Soil Temperature Matters More Than The Calendar
Nasturtiums are sensitive to cold soil. They germinate best when the soil temperature is consistently above 55°F (13°C). If the soil is still cold from a late spring, the seeds will likely rot. Use a soil thermometer to check. Stick it two inches deep in the garden bed. Wait until the reading stays above 55°F for three days in a row.
How To Warm Soil Faster In Wisconsin Gardens
If you want to plant earlier than mid-May, try these tricks to warm the soil:
- Cover the bed with black plastic for two weeks before planting
- Use a cloche or cold frame to trap heat
- Plant in raised beds, which warm up quicker than ground soil
- Choose a south-facing spot that gets full sun all day
Starting Nasturtium Seeds Indoors In Wisconsin
You can start seeds indoors 4-6 weeks before the last frost date. For southern Wisconsin, that means starting around early April. For northern Wisconsin, mid-April is safer. Use peat pots or biodegradable containers because nasturtium roots don’t like being disturbed during transplanting.
Steps for indoor starting:
- Fill pots with seed-starting mix
- Plant one seed per pot, 1/2 inch deep
- Water lightly and cover with plastic wrap
- Place in a warm spot, around 65-70°F
- Remove plastic once seedlings appear (7-10 days)
- Move to a sunny window or under grow lights
- Harden off plants for 7-10 days before moving outside
Direct Sowing Nasturtium Seeds In Wisconsin Soil
Direct sowing is the easiest method for most Wisconsin gardeners. Wait until after the last frost and the soil is warm. Soak the seeds in water for 12-24 hours before planting. This softens the hard seed coat and speeds up germination.
Direct sowing steps:
- Prepare the bed by loosening soil to 6 inches deep
- Plant seeds 1/2 to 1 inch deep
- Space seeds 10-12 inches apart for bush types
- Space seeds 18-24 inches apart for trailing types
- Water gently after planting
- Keep soil moist but not soggy until seedlings appear
- Thin seedlings to the strongest one per spot if needed
Wisconsin Growing Zones And Their Planting Windows
Wisconsin spans USDA hardiness zones 3b through 5b. Your exact planting window depends on which zone you live in. Here is a breakdown by zone:
Zone 3B And 4A (Northern Wisconsin)
These zones cover areas like Ashland, Bayfield, and parts of Vilas County. The last frost date is typically late May to early June. Plant nasturtium seeds outdoors around June 1-10. Indoor starting can begin around April 15-20.
Zone 4B (Central Wisconsin)
This zone includes Wausau, Stevens Point, and Wisconsin Rapids. Last frost usually occurs around May 15-25. Plant seeds outdoors between May 20 and June 5. Start indoors around April 1-10.
Zone 5A And 5B (Southern Wisconsin)
Madison, Milwaukee, and Kenosha fall into these warmer zones. Last frost happens around May 5-15. Plant outdoors from May 10 to May 25. Start indoors from late March to early April.
How Weather Patterns Affect Nasturtium Planting In Wisconsin
Wisconsin weather is unpredictable. A warm April can trick you into planting early, then a cold snap hits in May. Always watch the 10-day forecast before planting. If temps are predicted to drop below 40°F, delay planting or cover seedlings with row covers.
Signs That It’s Safe To Plant Nasturtiums
- Nighttime temperatures stay above 50°F for a week
- Soil feels warm to the touch (not cold and damp)
- Oak trees have fully leafed out
- Lilacs are blooming in your area
- No frost is forecast for the next two weeks
What Happens If You Plant Too Early
Planting nasturtiums in cold Wisconsin soil leads to several problems. Seeds may rot before they germinate. Seedlings that do emerge can be stunted by cold stress. A late frost can kill young plants outright. Even if plants survive, they often bloom later than those planted at the right time.
Best Nasturtium Varieties For Wisconsin Gardens
Some nasturtium varieties handle Wisconsin’s short growing season better than others. Choose varieties that bloom quickly, usually within 50-60 days from seed. Here are top picks:
Dwarf Or Bush Types
- Alaska Mix – Variegated leaves, early blooms
- Jewel Mix – Compact plants, flowers in 55 days
- Whirlybird Mix – Upward-facing flowers, no deadheading needed
- Tom Thumb – Very small plants, good for containers
Trailing Or Climbing Types
- Empress of India – Deep red flowers, dark leaves
- Moonlight – Pale yellow, long vines up to 6 feet
- Spitfire – Bright orange, vigorous grower
- Climbing Mixed – Good for trellises and fences
Preparing Wisconsin Soil For Nasturtiums
Nasturtiums prefer poor to average soil. Too much nitrogen makes them grow lots of leaves but few flowers. In Wisconsin, many gardens have clay soil or sandy soil. Both can work with a little adjustment.
Soil Preparation Steps
- Test soil pH – Nasturtiums like pH 6.0-7.5
- Amend clay soil with compost to improve drainage
- Add sand to heavy clay if drainage is poor
- For sandy soil, mix in a little compost to hold moisture
- Do not add fertilizer high in nitrogen
- Rake the bed smooth before planting
Container Planting For Wisconsin Patios
If your garden soil is heavy clay or stays wet, grow nasturtiums in containers. Use pots at least 8 inches deep with drainage holes. Fill with a well-draining potting mix. Do not use garden soil in containers. Place pots in full sun and water when the top inch of soil feels dry.
Caring For Nasturtiums After Planting In Wisconsin
Once your nasturtiums are in the ground, care is simple. They are low-maintenance plants that thrive on neglect. Overwatering and overfeeding are the most common mistakes.
Watering Guidelines
- Water deeply once a week if there is no rain
- Let soil dry out between waterings
- Water at the base to avoid wetting leaves
- Reduce watering in cool, rainy weather
- Container plants may need water every 2-3 days
Fertilizing Tips
Nasturtiums do not need fertilizer. In fact, fertilizing reduces flower production. If your soil is very poor, add a light layer of compost at planting time. Do not use liquid fertilizers or slow-release granules. Let the plants fend for themselves.
Pest And Disease Management In Wisconsin
Aphids love nasturtiums. In Wisconsin, aphids appear in mid-summer. A strong spray of water knocks them off. Ladybugs also eat aphids. Do not use chemical pesticides, as they kill beneficial insects.
Common diseases include powdery mildew, which shows up in humid Wisconsin summers. Prevent it by spacing plants for good air flow. Water at the soil level, not overhead. If mildew appears, remove affected leaves.
Harvesting Nasturtiums In Wisconsin
Both flowers and leaves are edible. They have a peppery, radish-like taste. Harvest flowers when they are fully open, usually in the morning. Leaves can be picked at any size. Use them fresh in salads, as garnishes, or in vinegars.
When To Harvest
- Flowers appear 50-60 days after planting
- Pick flowers regularly to encourage more blooms
- Harvest leaves before they get too large and tough
- Stop harvesting 4-6 weeks before first fall frost
Extending The Nasturtium Season In Wisconsin
Wisconsin’s first frost usually hits in late September or early October. You can extend the season by a few weeks with simple methods.
Fall Protection Tips
- Cover plants with row covers when frost is forecast
- Move container plants indoors to a sunny window
- Mulch around plant bases to protect roots
- Harvest all flowers and leaves before a hard freeze
Saving Nasturtium Seeds For Next Year
Let some flowers go to seed in late summer. The seeds are large and easy to collect. Wait until they turn brown and fall off the plant. Dry them indoors for a week, then store in a paper envelope in a cool, dark place. They remain viable for 3-5 years.
Common Mistakes When Planting Nasturtiums In Wisconsin
Even experienced gardeners make these errors. Avoid them for better results.
Mistake 1: Planting Too Early
This is the most common problem. Wisconsin gardeners get eager after a long winter. Resist the urge. Cold soil kills seeds. Wait until the soil is warm and the frost risk is gone.
Mistake 2: Overwatering
Nasturtiums are drought-tolerant once established. Too much water causes yellow leaves and root rot. Water only when the soil feels dry an inch below the surface.
Mistake 3: Overfertilizing
Rich soil produces huge leaves and few flowers. Plant nasturtiums in poor soil and skip the fertilizer. They bloom best when they are a little stressed.
Mistake 4: Planting In Shade
Nasturtiums need at least 6 hours of direct sun daily. In shade, they get leggy and produce few flowers. Choose a sunny spot in your Wisconsin garden.
Frequently Asked Questions About Nasturtium Planting In Wisconsin
Can I Plant Nasturtium Seeds In Wisconsin In July?
Yes, but it is risky. July planting gives you flowers in late August or September. A early frost could kill plants before they bloom. For best results, plant by mid-June at the latest.
Do Nasturtiums Come Back Every Year In Wisconsin?
No, nasturtiums are annuals in Wisconsin. They cannot survive winter temperatures. You must replant seeds each spring. They do self-seed occasionally if you let flowers go to seed.
How Deep Should I Plant Nasturtium Seeds In Wisconsin Soil?
Plant seeds 1/2 to 1 inch deep. In heavy clay soil, plant closer to 1/2 inch. In sandy soil, plant 1 inch deep. Cover lightly with soil and water gently.
Can I Start Nasturtium Seeds Indoors In Wisconsin?
Yes, but use biodegradable pots. Nasturtium roots are sensitive to transplant shock. Start seeds 4-6 weeks before the last frost. Harden off plants slowly before moving them outside.
What Is The Best Time To Plant Nasturtium In Wisconsin For Fall Blooms?
For fall blooms, plant seeds in early July. This gives plants 60 days to flower before the first frost. Keep plants well-watered during hot summer weeks. Choose fast-blooming varieties like Whirlybird or Jewel Mix.
Final Tips For Success With Nasturtiums In Wisconsin
Growing nasturtiums in Wisconsin is easy if you follow a few simple rules. Wait for warm soil. Plant in full sun. Do not fertilize. Water sparingly. Harvest flowers often. With these steps, you will have colorful blooms from mid-summer until the first frost.
Remember that Wisconsin’s weather varies from year to year. A warm spring might let you plant in early May. A cold spring might push planting into June. Always check your local forecast and soil temperature before planting. Your nasturtiums will reward you with bright flowers and tasty leaves all season long.
If you have heavy clay soil, consider raised beds or containers. If you have sandy soil, add a little compost to hold moisture. Nasturtiums are forgiving plants that adapt to most conditions. Just give them warmth, sun, and neglect, and they will thrive in your Wisconsin garden.
Start planning your nasturtium planting now. Mark your calendar for mid-May to early June. Prepare your soil in advance. Soak seeds overnight before planting. Water only when needed. Enjoy the show from July through September. With the right timing, your nasturtiums will be the highlight of your garden.