Using a greenhouse is the most reliable method of germinating hemp seeds for outdoor production. Temperature, humidity, and soil moisture levels are much easier to regulate, and supplemental lighting can be added to lengthen days during the spring. When seedlings sprout, they are immediately hardened off to the intense rays of the sun but still sheltered from the elements. With this in mind, we strongly discourage starting seeds completely indoors away from natural sunlight. Seedlings will grow spindly and weak, and suffer sunburn when moved from indoors to out.
Make sure to start out with a clean greenhouse and ensure no rodents call it home. Mice love hemp seeds. Unkempt greenhouses can also be the source of insect invasions (e.g. spider mites, russet mites, etc.) that may not show up in high numbers until later on in the season.
Keep it warm
Temperatures inside need to be 68 degrees and above for fast and consistent germination rates. We allow ours to reach upwards of 90 degrees during germination, but lower temperatures to between 60-80 after seeds sprout. Propane heaters are ideal and fans to circulate the air are highly recommended for both temperature control and to help build plant stalk strength. If you have tables to start seeds on this can speed up the germination process (if it's hot in your greenhouse) and keep you from having to bend over all the time. Humidity helps to both keep the soil moist and speed up vegetative growth.
Light it up
Supplemental lighting allows for plants to grow vigorously even during inclement spring weather. We use 10 Gavita DE 1000W lights in each of our 30x100’ greenhouses in the early morning and evening to ensure 16 hours of continuous light on seedlings (to prevent pre-flowering), and sometimes all day during cloudy spells. This also prevents seedlings from growing spindly and weak. CFLs and LED light strings can work if power is limited, but are not recommended.
If you plan to start seeds after the beginning of June, lighting is not necessary as days are longer. As long as plants are moved into their field environment prior to reaching sexual maturity (1-2 months old) they should have no problems adapting to altered light cycle when going from inside to out.
Start on a schedule
When to start your seeds should be based on when your field will be ready to be planted, and how long you intend to keep them growing in your greenhouse. We don’t recommend planting seeds in the field before June 1, as days are still short enough that plants may pre-flower. This greatly reduces yield. We start seeds at the beginning of May, about a month before we intend to plant. This allows for around 3 weeks of solid vegetative growth. The longer plants spend in greenhouses the more work it is for the farmer to keep plants watered and healthy. Greenhouses are also an easy all-you-can-eat buffet for insect pests.
Choose your size wisely
Choosing what kind of containers to sprout your seeds in should be based upon the space you have available, and how long they will remain inside prior to planting.
We use 50 and 72 cell vegetable starter trays with depths of 3” and cells between 1-2" squares. These trays hold around 4-5 quarts of soil and provide plants with enough space to grow for a few weeks before becoming root bound. Each unit covers about 1.25 sqft and with 72 cell units, 90,000 seeds can fit comfortably in a 30x100’ greenhouse.
Due to how quickly plants grow and fill their spaces, we do not recommend using trays that have more than 72 cells. The smaller the cell, the less time you have before you need to get plants into the field. We also don't recommend up potting plants due to added risk of plant shock and the added work and space required to do so. Moving thousands of plants into the field is much easier when they are in 50-72 cell trays vs larger pots.
We recommend using a fertile and well-draining potting soil. Ensure the soil is fine and all chunks are broken up. If you use a seed starter mix be aware it is generally devoid of nutrients and plants may yellow quickly from lack of nitrogen. Plan to fertilize. Using a potting soil with nutrients is recommended, or you can lightly amend a basic seed starter mix. On the other hand, if your soil is too rich in nutrients, seedlings burn easily. Finding a happy medium or planning to feed is your best bet. We discourage using peat moss plugs (a.ka. Jiffy Pellets) for starting seeds as they have almost no nutrients in them. Plants will either need to be fed constantly or transplanted into a medium with soil shortly after germination.
Don't plant too deep
We pre-poke ¼” holes in soil filled trays and place 1 seed in each, returning to cover them again lightly with soil. Making a mark on a kebab skewer is an easy tool for poking holes and can help keep you from burying seeds too deep. Remember to label all your trays.
Water in trays gently to prevent seeds from floating to the surface. Mister attachments for hoses work great. Keep the soil moist, not soggy, and don’t let it dry out. This is one of the most critical parts of the season, so keep an eye on your trays! Seedlings will sprout up within days in ideal conditions, but can take up to several weeks for all seeds to pop. If conditions are less than ideal germination rates will be sporadic and you may have to wait up to 3 weeks for sprouting.
Within a week or two of soil emergence, seedlings can drink up water very quickly and may need to be watered several times a day. Having humidity levels around 60% allows them to absorb some water through their leaves and help plants thrive.
Feed when in need
We recommend feeding plants a liquid fish emulsion after they've been above ground for about 2 weeks, or if leaves are yellowing from malnutrition. Start with half dose of what the label recommends as seedlings are delicate, and repeat weekly until ready to field plant. If your planting date is pushed back by field preparations and seedlings become root-bound, foliar feeding can help keep plants healthy. Always start light.
Get seedlings out into the field when they are 6-10” tall. If they grow too tall and lanky, they will be susceptible to blowing over in the field. If left for too long in their containers they will become root bound and suffer from greater shock when transplanting. This can greatly slow their rooting in their new home and set growth back by a few weeks. Avoid planting in 100+ degree heat. If the weather is hot and sunny we recommend early mornings and late afternoon plantings if possible.
Don't wait too long to plant
You can find more tips in our cultivation guide, downloadable here.