3As Indiana enters the prime transplanting window for warm-season crops, vegetable producers must prepare transplants for successful field establishment. With variable spring weather conditions across the state, proper hardening and planting techniques are essential for minimizing losses and ensuring quick establishment. This article outlines critical management practices for growers preparing vegetable transplants for field conditions.
Hardening Transplants: Essential for Field Success
Why Hardening Matters
Properly hardened transplants show significantly higher survival rates and faster establishment in field conditions. The hardening process builds carbohydrate reserves, promotes root development, and thickens cell walls—all of which are critical for transplants to withstand field stresses. For growers, even a slight percentage improvement in transplant success can translate to substantial yield increases.
Practical Hardening Steps
- Begin 4-8 days before field planting
- Day 1-2: Place transplants outdoors in 50% shade for 3-4 hours when temperatures are at least 50°F
- Day 3-4: Move to 20% shade, increasing outdoor exposure by 1-2 hours daily
- Day 5-6: Transition to full sun exposure during daytime hours
- Day 7-8: Leave plants outdoors permanently if nighttime temperatures remain above 50°F
- Irrigation management during hardening
While reducing irrigation is part of the hardening process, operations should use careful monitoring:
- Gradually decrease irrigation frequency but maintain adequate moisture
- Use soil moisture sensors or weight-based monitoring for consistency
- Ensure complete saturation of the growing medium when irrigating
- Never allow plants to reach wilting point
Simple Hardening Methods
For those without specialized equipment:
- Use a shade tree to provide filtered light
- Move plants to a cart or wagon that can be easily relocated
- Cold frames work well for smaller operations
Timing Your Transplanting
Minimum Temperature Requirements
- Cool season crops: 40°F air / 50°F soil
- Warm season crops: 50°F air / 60°F soil
Check Indiana-Specific Resources
- Use the Indiana Mesonet (campbellcloud.io/stations.php) for current soil temperatures
- Consult Purdue’s frost probability maps (https://mrcc.purdue.edu/freeze/freezedatetool) before planting frost-sensitive crops
Ideal Conditions for Transplanting
- Humid, overcast, and calm days
- Adequate soil moisture
- Morning or evening hours rather than midday
Managing Delayed Transplanting
When weather or other events don’t allow planting at the desired growth stage.
Holding Transplants Successfully
- Prevent plants from overgrowing while keeping them healthy
- Use the same irrigation and fertility approach as during conditioning
- Monitor for nutrient stresses and add fertility as needed
- Hold transplants preferably in a well-ventilated shade house
This “holding” technique helps when spring storms, equipment breakdowns, or labor shortages delay your transplanting schedule. The key is preventing transplants from becoming rootbound or stressed while maintaining their hardened condition.
Starter Fertilizer: Worth the Investment
Starter fertilizer provides readily available nutrients right at the root zone, especially important in:
- Cool spring soils with limited phosphorus availability
- Systems relying on organic nitrogen, where some available nitrogen may be helpful in cool soils
Application Options
- Apply to seedlings while still in flats shortly before transplanting
- Include in the transplant water
- Mix into the transplant hole at planting time
Transplanting Best Practices
Before Heading to the Field
- Water transplants thoroughly before taking them to the field
- Bring extra water to keep trays moist while working
Proper Planting Technique
- Handle seedlings by the root ball or leaves, not the stem
- Plant at the correct depth:
- Always cover root ball and growing medium completely
- Tomatoes: can be buried up to first true leaf
- Peppers: can be buried up to first true leaf
- Cucurbits: plant up to but not over the cotyledons
- For grafted plants: keep graft union above ground
- Ensure good soil contact with the root ball
- Water in immediately with 8 oz. of starter solution or plain water per plant
Plastic Mulch Considerations
- Make sure the seedling stem doesn’t rub against the mulch hole edges
- Risk of stem damage increases in windy conditions or high temperatures with black plastic
After Transplanting: First Critical Days
- Check plants within a few days of transplanting
- Scout for pests, especially cutworms
- Replace failed plants within one week to maintain uniform harvest
- Provide protection from strong winds and frost if needed
- Be prepared to irrigate if dry conditions persist
The Bottom Line: 10 Keys to Success
- Harden off transplants properly (4-8 days)
- Use starter fertilizer
- Time planting according to soil and air temperatures
- Water well before taking plants to the field
- Handle seedlings carefully
- Plant at the correct depth
- Water in thoroughly after planting
- Protect from environmental stresses as needed
- Check plants within a few days and replace failures promptly
- Manage plant stress during the first 1-2 weeks after transplanting
Consider Reading the Following Literature
Setting Your Transplants Up for Success in VCH issue 717
Start Your Season off Right with Pest-free Transplants in VCH issue 715
A Look at the Freeze Date Tool from Midwestern Regional Climate Center in VCH Issue 713
Protect Early Planted Warm Season Vegetables in VCH Issue 672