Cucumbers need very warm soil to germinate. If direct sowing, wait until mid-June. If weather turns cool and wet after that, just re-sow. Or start transplants indoors in individual peat or coir pots 3-4 weeks before transplanting out into warm soil. If starting indoors, use bottom heat. Transplant when the plants develop their third true leaf. If the plants are too big, they may experience transplant shock. Optimal soil temperature for germination (and transplanting): 15-30°C (60-85°F).