Knowledge Base

A quick-reference germination guide for all major seed categories carried by Paramount Seeds — including tomatoes, cucumbers, peppers, lettuce, herbs, and more.

Learn what seed priming is, how it works, and why it matters for greenhouse vegetable production. Discover the benefits of primed seed for tomatoes, rootstocks, and more — with examples from the Paramount Seeds catalog.

Greenhouse cucumbers are one of the most rewarding crops to grow — but getting germination right is the foundation of a successful crop. Whether you're working with treated or untreated seed, the right environment and technique will give you fast, uniform sprouting and strong transplants. Here's a step-by-step guide to getting it right. What You'll Need Greenhouse cucumber seed (treated or untreated) Rockwool cubes or coco coir propagation plugs Seedling tray with humidity dome pH-balanced water (pH 5.8–6.2) Bottom heat mat Grow lights (LED or HPS) Thermometer and hygrometer Step 1: Pre-Soak Your Growing Medium Soak rockwool cubes in pH-adjusted water (pH 5.8–6.0) for at least 30 minutes before sowing. Rockwool is naturally alkaline — skipping this step can cause nutrient issues right from the start. Shake out excess water gently. The medium should be evenly moist, not dripping wet. Step 2: Set Up Your Germination Environment Before sowing, get your environment dialed in. Cucumber seeds need warmth to germinate reliably: Substrate temperature: 77–86°F (25–30°C) — use a bottom heat mat Air temperature: 72–77°F (22–25°C) Relative humidity: 90–95% under the dome Cold conditions below 60°F will cause slow, uneven germination or seed rot. Step 3: Sow the Seeds Place one seed per plug, about ½ inch (1–1.5 cm) deep, with the pointed end facing down. Cover trays with a humidity dome immediately after sowing to lock in moisture and heat. Under optimal greenhouse conditions, expect to see radicles emerging in 2–4 days. Step 4: Maintain Moisture Check trays at least twice daily. The growing medium must stay consistently moist — if it dries out, germination rates will drop. Mist lightly if the surface looks dry, but avoid waterlogging, which promotes rot. Step 5: Remove the Dome at Emergence As soon as the first cotyledons appear, remove the humidity dome. Leaving it on too long in a warm greenhouse creates the perfect conditions for damping-off and fungal issues. Increase airflow gradually to harden seedlings to greenhouse conditions. Step 6: Provide Light Once seedlings emerge, move them under your grow lights immediately. Cucumbers need 16–18 hours of light per day at 200–400 µmol/m²/s to develop compact, vigorous transplants. Insufficient light leads to stretchy, weak seedlings. Step 7: Transplant at the Right Time Seedlings are ready to transplant when they have 1–2 true leaves and a well-developed root system — typically 10–14 days after sowing. Don't let them become root-bound in the plug; transplant shock is harder to recover from at that stage. Tips for Better Germination Rates Use bottom heat — substrate temperature matters more than air temperature for cucumber germination Don't overwater — moist, not wet, is the goal at every stage Use sterile media and clean trays — reusing old trays without sanitizing is a common source of Pythium Label your trays — if trialing multiple varieties, keep them organized from day one Troubleshooting No germination after 5 days — Check substrate temperature (too cold?) and moisture (dried out?) Leggy seedlings — Lights too far away or too few hours; move lights closer and increase duration Damping-off or rot — Humidity dome left on too long or overwatering; improve airflow and reduce moisture Uneven germination — Temperature fluctuations; check heat mat consistency across the tray Ready to Grow? Browse our popular greenhouse cucumber varieties — bred for high performance, disease resistance, and consistent yields in controlled environments: Verdon F1 Long Dutch Cucumber (Treated) Bologna F1 Long Dutch Cucumber Georgia F1 Long Dutch Cucumber (Treated) Excelsior F1 Pickling Cucumber (Untreated) Corinto F1 Slicer Cucumber Have questions? Get in touch with the Paramount Seeds team — we're happy to help you find the right variety and get your crop off to a great start.