Tips to Grow Great Seedlings at Home

By Jeremy DeLisle, UNH Cooperative Extension Education Center Coordinator, with contributions from Tom Kamberis, New Hampshire Master Gardener

Q: I’d like to grow some of my own transplants for the garden this year. Can you provide some tips that will help get me started on the right track? — Richard, Gilmanton

A: When planning to germinate your own vegetable, fruit, or flower seeds, three major factors should be considered first and foremost: soil moisture, temperature, and light.  

Choose a Mix Specifically for Starting Seeds

Always select a mixture specifically labeled for seed starting. This will ensure the soil’s ability to hold moisture while allowing enough drainage to prevent waterlogged seeds. Typical mixture ingredients include peat moss or coconut coir, perlite, vermiculite, potentially compost, worm castings and other sources of conventional and organic fertility.

Maintain Adequate Moisture and Temperature

Pre-moisten the mixture before seeding and maintain a steady supply of moisture while seeds are germinating. The soil mixture should feel like a damp sponge, and squeezing a handful should produce a few drops of water. Trays outfitted with plastic domes or covered with plastic wrap can keep the mixture moist during germination. Be sure to remove the cover as soon as the seeds sprout to allow ventilation and prevent conditions that encourage disease. 

Ideal germination temperature ranges vary from seed to seed, but most seeds will germinate well at a soil temperature 65–75 degrees. Heat mats with a simple temperature probe and thermostat will increase success substantially and are well worth the investment. 

Provide Adequate Light

Seed packets indicate whether the seeds require light to germinate. Cover seeds that need light with a dusting (less than ¼ inch) of peat moss or vermiculite to conserve moisture and prevent the seeds from washing out when you water.

Supplemental light is often required for growing good quality transplants. An inexpensive shop light with cool white or full spectrum bulbs will work fine. 

You will want to start out with the bulbs just 3 to 4 inches away from the seedling and maintain that distance as they grow. It is key to raise the light as seedlings grow and ensure that the light is available as soon as the seedlings emerge to prevent stretched out or “leggy” plants. 

These lights should be left on for 16 hours per day to ensure adequate light for growth. Programmable timers can be used to turn lights on and off.  

Ordering Seeds

Time is of the essence where placing seed orders is concerned.  Cold-hardy crops like kale could be started as early as the first couple of weeks in March to produce a six to eight week old transplant ready to be planted into the garden around May 1st. For transplants with a target planting date of Memorial Day, we have an extra month or so before many of those should be started. 

Ordering early helps ensure that the varieties at the top of your list are still in stock.  Buy quality seed from reputable companies. Try to learn a little about the companies that you buy from, as breeding programs and proper maintenance of varieties can make a huge difference in the success and final products of your garden. 

Seeds ordered this season should be marked with the date stating that they were packaged for sale in 2017.  While seeds can be kept for more than one year, viability of seeds tends to decrease over time. Viability charts for a variety of garden seeds are available online. Ordering only what you will use each year and using fresh seed will help increase success with germination rates. 

Seed packs saved from year to year should be resealed and stored in a cool, dark location after use. To check older seeds for viability, you can conduct a simple germination test by planting a small number into a seed starting mix. An aluminum pie pan works well for this purpose, and can hold multiple batches of seed.  Ten seeds is a good number to try, since all ten germinating represents a 100% germination rate, while five would represent 50%. Even if only half of your seeds germinate, they can still be used.  Simply seed at twice the recommended rate on the seed packet, knowing that only half of the seeds are likely to sprout. 

Follow these tips and you’ll be on your way to a great start to the gardening season. For more information, refer to UNH Cooperative Extension’s Starting Plants Indoors from Seed.

UNH Cooperative Extension is the educational outreach component of the University of New Hampshire. Extension’s Education Center answers questions about gardening and more at or by calling 877-398-4769 Monday through Friday, 9 a.m. to 2 p.m.