Shipping Seeds & Seedlings in Low Temperatures

Cold weather can quietly destroy seed viability and shock tender seedlings. Whether you’re a nursery, seed company, or home gardener shipping live starts, proper temperature management ensures your plants arrive healthy and ready to grow. This guide explains how to protect your shipments with UniHeat shipping warmers, designed for safe, oxygen-activated warmth originally made for live plants, insects, and animals.

1) Understand What Cold Does to Seeds and Seedlings

While dormant seeds seem resilient, extended cold exposure during transit can lower germination rates. For young seedlings, temperature drops can cause cell damage, wilting, or death within hours. Even brief exposure to freezing air on airport tarmacs or in delivery trucks can ruin a shipment.

• Seeds: Below 40°F, moisture inside the seed coat expands and ruptures delicate internal tissues.
• Seedlings: Exposure to cold air can stop photosynthesis and cause blackened leaves or root shock.
• Soil plugs: Retain cold for hours, acting as a heat sink that prolongs chilling effects.

2) Use UniHeat Warmers to Maintain Root-Zone Temperature

UniHeat warmers produce consistent, low-level heat ideal for live plants and seedlings. Their oxygen-activated design provides breathable warmth without scorching delicate roots. Unlike hand warmers, which can exceed 130°F, UniHeat maintains a gentle 100°F maximum—perfect for boxes containing live greens.

• 40-Hour UniHeat Pack — Use for 1-day routes or mild fall weather.
• 72-Hour UniHeat Pack — Ideal for 2–3 day shipments with insulated boxes.
• 96-Hour UniHeat Mega Warmer — Best for long-distance or cold-climate deliveries.

For most seedling boxes, one pack per cubic foot of insulated space maintains ideal warmth (between 60–75°F) throughout transit.

3) Build a Breathable, Insulated Box Setup

Plants need both warmth and air. Over-insulating or sealing the box too tightly can suffocate the heat pack. Follow this layout for consistent results:

1. Outer carton: Corrugated box rated for at least 200# burst strength.
2. Inner liner: Foam or reflective bubble wrap to slow heat loss.
3. Vent holes: Two small pencil-size holes on opposite sides of the outer box to allow oxygen flow.
4. Heat pack placement: Tape the UniHeat pack to the lid or side panel with cardboard spacing—never in direct contact with roots or leaves.

Fill empty spaces with crumpled paper or breathable cushioning. Avoid sealed plastic bags unless necessary for humidity control.

4) Time Shipments to Avoid Weekend Delays

Even with the right setup, delayed delivery can drop internal temperatures dangerously low. Schedule plant and seed shipments between Monday and Wednesday to prevent weekend holds at unheated facilities.

• Choose 1–3 day services (Priority or Express).
• Confirm carrier hub temperatures for cross-country shipments.
• Notify receivers to bring packages indoors immediately upon arrival.

It’s far safer to delay shipment one day for warmer weather than risk a full crop loss due to freezing exposure.

5) Label Clearly and Educate Customers

Inform customers that your shipments are temperature-sensitive and require prompt handling. Use “Live Plants – Protect from Cold” or “Temperature Sensitive” stickers on every carton.

• Include unpacking instructions inside (e.g., “allow seedlings to acclimate indoors for 12–24 hours”).
• For wholesale shipments, attach a packing slip with the activation time of the heat pack.
• Train fulfillment staff on how to safely pre-activate and position UniHeat packs.

Frequently Asked Questions

Do all plant shipments need heat packs in winter?

Yes—if temperatures anywhere along the route fall below 55°F. Even tropical seeds and dormant roots can lose viability after cold exposure.

Can I ship seeds and live plants in the same box?

Yes, but ensure seedlings are insulated from direct heat. Place the UniHeat pack on the side nearest the seeds or under a cardboard divider.

Are UniHeat packs safe for organic or edible plants?

Yes. UniHeat warmers are non-toxic and contain natural iron powder, vermiculite, and water—safe for use around all plant material.

How long should I pre-activate the pack?

Activate the warmer 20–30 minutes before sealing. It should feel gently warm (not hot) before placing it inside the insulated box.

What duration should I choose for long-distance shipments?

Use a 96-hour UniHeat Mega Warmer for shipments expected to take more than 3 days, especially during late fall or winter.

Final Takeaway: For every live plant or seed shipment, balance insulation, ventilation, and heat duration. Protect your investment and customers’ growing success with UniHeat’s 40hr, 72hr, and 96hr packs, available at UniHeat.us.