By Steve Reiners, Professor and Chair, Horticulture Section, School of Integrative Plant Science, Cornell University, Cornell AgriTech. This is part of a series of columns that he wrote about vegetable gardening during the pandemic.
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April 2, 2021

Should you plant seeds outside? Or start them inside? Or purchase plants to transplant? It depends on the crop.

Last week I mentioned that one of the biggest mistakes gardeners make is starting their transplants too early.  I just had  a colleague ask what she should do with the gourd plants she started as they are getting too big.  Gourds are a heat loving crop and should be planted outside only after the last chance of frost and after the soil is consistently warm.  That would be around June 1 here. Gourds, like squash and melons, only need about three weeks of a head start inside. The time to plant would be about May 10.  She started hers February 28!

The table below provides some guidance on when you should start your transplants. It assumes a last frost date of May 20.  If your last frost date is earlier or later, add to or subtract from the optimum planting dates to compensate for the difference.  Other than onions, there is still time to get these started indoors.

When to start vegetables indoors and optimum temperatures for growth

Now let’s talk about the other important factors when it comes to transplants – lighting, temperature, soil, and containers.

The most important is light. Even a sunny window facing south will not provide enough light both in quantity and quality to raise healthy transplants. You can create a light box with fluorescent lights suspended about 4 inches above the plants. You need to be able to adjust the height of your lights to maintain that 4 inch distance as your plants grow.

There are grow lights that you can buy that include wavelengths more beneficial for plant growth. Try to keep the lights on for 16 to 18 hours per day separated by 6- to 8-hour dark periods.

Follow the recommendations in Table 1 for optimum growing temperatures. Try to reduce the temperature by about 10F from the optimum at night, as this will help control plant size.

To speed germination, try keeping the temperature 10F above optimum from planting until your seeds sprout. Most seeds don’t require light to germinate so you can put your containers in a warm, dark place after seeding them. Then move them under the lights and dial back the heat once the seeds sprout. You can buy electric heating mats that will warm your containers, soil and seeds from the bottom.

For the soil, it’s best to use a good quality, already prepared sterile media that can be purchased at any garden center. These are typically prepared with peat moss and other materials such as perlite, lime, and a little fertilizer. Don’t use soil from your garden as it may contain some plant diseases and won’t drain well, making your seeds vulnerable to rotting in soggy soil.

You have lots of choices for containers. I prefer to use plastic cell-packs as they can be washed and reused for many years. But you can use paper or Styrofoam cups, egg cartons, and pots made of peat moss or composted manures. Whatever you use, make sure they have holes in the bottom so excess water can drain out.

Find more seed starting details in this Cornell Cooperative Extension factsheet: Indoor Vegetable Seed Starting.

There is a two-day, Cornell virtual gardening conference coming up April 27 and 28 that has been organized by the NYS Integrated Pest Management Program (IPM). “Vegetable IPM – Gardening from the Ground Up”. Experts from across New York, including me, will provide quick, 25 minute presentations each morning, from 8:30 a.m. to 12 noon, with a special virtual lunch chat on the first day where you can meet and ask questions of the speakers.

For more information on topics, speakers and registration, please visit the IPM website. I hope to see many of you there.

In the garden this week.

Lots of seeds can be started indoors now as long as you have the conditions highlighted above. You can plant peas, spinach and Swiss chard outside now. Cover with clear plastic to increase soil temperatures and speed germination. Clean up garden beds. If starting a new plot this year, have your soil tested. Check with your local Cornell Cooperative Extension office in your county for soil testing details.