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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March 26, 2021

Sometimes it can be a bit confusing when you’re trying to decide whether to use vegetable  transplants (starts) or plant seed directly in your garden.  There are advantages and disadvantages to each method and a lot depends on both the vegetable and the time of year.

If we lived in a nice, warm place with a long growing season, it would be easy to start everything in the garden with seeds.  But here in upstate New York, our growing season is limited to 130 to 150 days of frost-free growing.  Although that sounds like a long time, consider how chilly it can be even after the last spring frost.  Our soils remain pretty cold into June.

When you plant a seed into a cold soil, it germinates very slowly.  Even cold tolerant seeds like peas and spinach, which can be planted right now (late March or early April, depending on the season), germinate better when the soil is warmer.  For example, it would take peas about 35 days to germinate with a soil temperature of 41F.  Raise the temperature to a still cool 50F and it takes only 14 days.  Go to 60F and they emerge in 9 days.

Slow emergence in a cold soil exposes the seed to more plant diseases.  The seed may rot before it germinates. As of March 25, soil temperatures in Ithaca were close to 46F and in Geneva, about 50F.  You can speed things up  by laying a clear plastic sheet over the soil after planting to warm it up.  The heat from the sun will raise the soil temperature 10 to 15 degrees.  Once you see the seedlings emerging, remove the sheet.

Obviously, if cold-loving seeds like peas have a problem in cold soils, heat-loving crops like tomatoes and peppers would fair much worse.  Each requires a minimum of 60F and would prefer a soil ten degrees warmer. That’s why we start them indoors where we can keep temperatures warmer and transplant the starts outside later in the season after danger of frost has passed and the soil has a chance to warm up. 

As a gardener, you’re probably thinking, why don’t we use transplants for everything?  Unfortunately, some crops respond poorly to transplanting.  Beans and peas for example, often succumb to transplant shock and even those that survive will be weak and poor-yielding. 

Crops that we grow for their roots, like carrots, beets and turnips also don’t transplant well.  Often the large taproot is lost, and we’re left with a plant with a fibrous root system.  That’s fine for the plant but not good for us as we need that taproot to harvest.

There are some plants that will do fine planted either way, depending on the season.  Lettuce is a good example of that.  In the spring, when the soil is cool, it is best to use transplants.  If planting in late August for a fall crop, feel free to direct seed.  Lettuce seed will take two weeks to germinate in cold April soil but 3 to 4 days in the summer.  

Best ways to start vegetables for our gardens

Direct Seed Outdoors

• Beans
• Beets
• Carrots
• Parsnips
• Peas
• Radish
• Rutabagas
• Sweet corn
• Turnips

Direct Seed or Transplant

• Broccoli
• Cabbage
• Collard
• Cucumber
• Endive/Escarole
• Kale
• Lettuce
• Pumpkin
• Spinach
• Squash
• Swiss chard

Use Transplants Started Indoors

• Brussels sprouts
• Celery
• Eggplant
• Leeks
• Melons
• Pepper
• Tomatillo
• Tomato

One advantage of starting your own transplants rather purchasing them from a garden center is choice.  Even the best garden center may have no more than 15 or 20 varieties of tomatoes.  And likely far fewer of other, less-popular vegetables.  By purchasing seed and starting your own, you may have a choice of hundreds of unique varieties.

You might be thinking that with the warm spring weather we’ve been having that it is too late to start warm-season crops like tomatoes, peppers, eggplant, squash and cucumbers.  But one of the biggest mistakes gardeners new to starting their own seeds is starting them too early. 

You only need to start tomatoes and eggplant about 6 weeks before transplanting. So, if you are shooting to transplant them around June 1, you have until mid-April or so to start them inside.  Peppers can use a bit more time. Start them about 8 to 10 weeks before transplanting.  For extra early cucumber and squash harvest, start plants inside 2 to 4 weeks before transplanting outside after all danger of frost has passed. But use care when transplanting as their root systems are fragile.

Starting the seeds indoors sounds easy enough but it requires expertise and special conditions.  Gardeners often underestimate the light requirements of the plants and wind up with tall, spindly plants that don’t do well once taken outside.  Sometimes it’s easier to use a professionally grown plant.

In the garden this week

Peas, spinach and Swiss chard can be seeded in the garden.  Cover soil with clear plastic to increase soil temperatures and speed germination. Also clean up the garden and get rid of last year’s crops and weeds. Still too early to start most heat-loving crops like tomatoes and eggplants inside. (It’s about the right time to start peppers.) But leafy greens, and crucifers (cabbage family crops including broccoli, cauliflower and kale) can get started, assuming you have a well-lit and heated area in your home.