Seed Starting: Ready for Transplant?
- Suzanne Sauls
- May 27
- 5 min read
Welcome back to our seed starting blog series! You've nurtured your little seed babies for weeks--maybe even months--and now you're eager to introduce them to their new home in your garden. But when is the ideal time to transplant?
If you ask a Master Gardener, the most likely response will be: "It depends." And they're not just being coy! Transplant timing isn't an exact science--it's a delicate balance of environmental conditions and plant readiness.
In this article, we'll explore some key cues to help you determine whether your seedlings are ready for transplant and how to set them up for a smooth transition outdoors.
Environmental Cues: Is Your Growing Space Ready?
As we've covered in previous articles, plants thrive best in optimal growing conditions, and temperature plays a major role in transplant success. If the weather is too cold or too warm, some plants may struggle or even fail to thrive.
Most regional planting guides, such as the one below from the Cheyenne Botanic Gardens, provide seasonal estimates based on air temperatures indicating historical frost dates.
The University of Wyoming Laramie County Extension identifies May 20th as our average last frost date, and September 25th as our average first frost date.[1] But as you can see from the planting schedule above, some crops can be transplanted earlier than the last frost date--like asparagus, cabbage, and hardy annuals. These are "cool season" varieties, meaning they tolerate lower temperatures and actually benefit from an early start before the heat sets in. On the other hand, "warm-season" crops--including tomatoes, peppers, zinnias, and cosmos--prefer to wait until all risk of frost has passed before transplanting.
Check local conditions, not just charts! While planting guides are helpful, Spring temperatures can fluctuate, so we recommend checking your 10-day weather forecast before transplanting. And don't forget about soil temperature. Though often overlooked, soil temperature can significantly affect root development, water uptake, and overall stress tolerance. [2] Luckily, online tools like GreenCast Online's Soil Temperature Map and simple soil thermometers allow you to check conditions right in your own garden. Remember, different areas of your garden will warm up at different rates--due to shade, wind exposure, raised vs. ground beds--so be sure to test multiple spots.
GreenCast Online (In-Ground) Soil Temperature Graph for May 26, 2025 (57 degrees F) &
Raised-Bed Soil Temperature for May 26, 2025 (55 degrees F) (photo: S. Sauls)
Seedling Cues: Is Your Plant Ready?
Your seedlings will show you when they're ready for transplant. Pay close attention to these indicators:
Healthy Root Structure: A well-developed root system helps plants anchor into the soil allowing them to absorb water and nutrients and tolerate environmental stress.[3] Check that your seedlings appear well-established within their pots before transplanting.
Stem Height and Strength: Stems provide structural support, transporting nutrients and helping plants adapt to wind and other outdoor elements. Aim for moderate height (check your seed packet) with sturdy stems. Want to encourage stronger stems? Try placing a gentle fan near your seedlings as they grow indoors to mimic the natural wind conditions they'll experience outdoors. Some seedlings, like tomatoes, may benefit from burying their stems deeper at transplant time to further strengthen their root system.
True Leaves Have Emerged: When a seed germinates, the first tiny leaves you see ---called cotyledons--aren't true leaves. Instead, they store nutrients for the young plant. True leaves emerge shortly after cotyledons, resembling miniature versions of adult leaves. Most seed packets recommend transplanting only once one to three sets of true leaves have formed.
Examples: Are My Seedlings Ready for Transplant? Let's take a quick look at a couple of my seedlings:
Bullseye Geranium (Pelargonium) Seedling: I started these seeds on March 16, 2025. As of May 26, 2025, the plant height is approximately 3 inches, with a sturdy stem; there are at least three sets of true leaves on the plant; and the root structure appears well-established in its pot. Is it ready to transplant? Not yet. Although my seedling is ready, my environment is not. As of May 26, 2025, my 10-day weather forecast projects low temperatures between 40-50˚F, but the seed packet recommends nighttime temperatures in the 60s with soil temperatures ideally between 70-75˚F. I'll wait another week or two before considering transplant outside.
Geranium Seedlings -- as of May 26, 2025, the plant is ~3 inches tall, with at least three sets of true leaves, and the root structure appears well-established within its pot.; photo credit: S. Sauls (May 2025)
Patterson Onion Seedlings: These seeds were started April 11, 2025. The foliage height is currently about 4 inches, with a sturdy stem and roots beginning to emerge from the bottom of the seed tray. Patterson onions can tolerate soil temperatures of 55˚F and can withstand frosts in low 30s, which my 10-day forecast meets or exceeds. Ready to transplant? Yes! Even though these seeds were started later than the geranium, they will be transplanted earlier.
Patterson Onion Seedlings -- as of May 26, 2025, the foliage is ~4 inches tall, with roots starting to emerge from the seed tray bottom; photo credit: S. Sauls (May 2025)
Hardening Off: The Final Step
If your seedling is ready and your garden conditions are cooperating, the last step before transplanting is referred to as "Hardening Off." This gradual process (typically 1-2 weeks) helps seedlings acclimate to their final outdoor environment by slowly increasing their exposure to sunlight, wind, and varying temperatures. The table (below) suggests daily exposure activities for hardening off your seedlings.
Table - Hardening Off Schedule (suggested)
Day | Description |
Day 1 | Place seedlings outdoors in a shaded, wind-protected area for 2 hours. |
Day 2 | Increase outdoor time to 4 hours, still in the shade, with light wind exposure (near a fence or wall). |
Day 3 | Extend to 6 hours, allowing seedlings to experience gentle breezes. |
Day 4 | Move seedlings to a partially sunny spot for 8 hours, with moderate wind exposure. |
Day 5 | Increase exposure to 10 hours in full sun, with stronger wind exposure (open area). |
Day 6 | Leave seedlings outside for 12 hours, ensuring they get full sun and consistent wind exposure. |
Day 7 | Leave seedlings outside all day and bring them in at night, allowing them to adjust to natural wind conditions. |
Day 8 | Seedlings are ready for transplanting into the garden! |
Be Patient--Your Seedlings are Tougher Than They Look! Hardening off takes time, but don't worry--your plants will thank you for it. By easing them into outdoor conditions, you'll reduce transplant shock and set them up for a successful growing season.
A Final Word
Transplanting can feel a little nerve-wracking, but your seedlings have come this far under your care, and with a little patience, they'll thrive in their new home. Now that you know what to look for, take a moment to check in on your seedlings--are they showing signs that they're ready for the big move? If so, get excited! You're about to take a crucial step in your gardening journey.
Written by Suzanne Sauls, Laramie County Advanced Master Gardener
References & Resources
[1] University of Wyoming Extension Publication B-1340, Wyoming Vegetable & Fruit Guide, Table: “Probabilities of Last Spring or First Fall Temperature Occurring,” pages 4-5, Nove 2021, accessed May 6, 2025, at https://www.wyoextension.org/agpubs/pubs/B-1340.pdf
[2] Leeper, R.D., Matthews, J.L., Cesarini, M.S., & Bell, J.E. (2021). Evaluation of air and soil temperatures for determining the inset of growing season. Journal of Geophysical Research: Biogeosciences, 126, e2020JG006171. Accessed May 20, 2025, at https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020JG006171
[3] Rizwan Rashid Bazmi, Pharkphoom Panichayupakaranant (2023). “The Role of Roots, Stems, and Leaves in Plant Function: Structural and Physiological Perspectives for Optimized Plant Growth,” Australian Herbal Insight, 6(1), 1-5, 9956. Accessed May 2025 at https://publishing.emanresearch.org/CurrentIssuePDF/EmanPublisher_3_5778ahi-619956.pdf.
"Garden Tips from the Cheyenne Botanic Gardens: Cheyenne Planting Schedule," Version 2, 2010, accessed May 26, 2025, at https://www.botanic.org/wp-content/uploads/2018/01/Cheyenne-Planting-Schedule.pdf
