Garden advice: The best time to transplant seeds

Question: I’ve been reading articles about how to start seeds indoors before planting them outdoors. How do I distinguish a “true leaf” from a "seed leaf?" The directions say to transplant things when they have 2 to 3 sets of “true leaves.” I have not been able to find this information in any of my gardening books, even though, they too, use the term “true leaf.”

Answer: Plant scientists call the very first leaf or leaves that show up when a seed germinates “cotyledons” or “seed leaves.” These seed leaves came out of the seed along with the plant embryo. These seed leaves provide nutrients for the tiny sprouting embryo plant as it begins to grow. 

Some seeds such as beans contain relatively big seed leaves. Next time you plant beans, watch as they germinate. Those first fleshy leaves are the seed leaves, or cotyledons, as they were part of the seed. The next pair of leaves that grow are thinner and more defined. These the first of the “true leaves.”

When a seed germinates, it is racing to produce its own true leaves before the food stored in the seed leaves runs out. If you plant old seed, germination and growth are not as vigorous as when you plant fresh seed with fresh seed leaves for the growing sprout.

Old seeds often have depleted seed leaves. They lose their vigor during long storage. Think of it as if you are feeding your baby plants rotten leftovers from the fridge instead of fresh mother’s milk. 

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Once your seeds are up, be careful not to harm the seed leaves when watering, thinning or transplanting. If the seed leaves are destroyed or damaged early on, it dooms the plant.

Err on the side of waiting for warmth. If you plant seed outdoors too early in soil that is too cold and wet, the seed leaves may exhaust themselves before the plant produces enough true leaves to grow vigorously. 

Pay attention to soil temperature. If temperatures of your soil (not your air in the yard) stay below 50 degrees, then you may have to wait, use techniques such as black plastic or a cold frame or plant sprouted seed.

Here are a few more secrets for beating cold and wet spring conditions: 

• Presprout peas by soaking them for a few hours in water, then start them indoors in newspaper tubes, toilet paper roll cardboard or paper cups. When they are a couple inches tall, cold harden them for a week or more. Then plant them outside when they are 4 to six inches tall. 
• Protect new sprouts from birds and other critters, as these animals love to eat the young pea plants. Spun row cover or chicken wire are good tools to keep marauding birds away. 
• Use a cold frame or plastic hoop houses over your beds to keep the rain off young transplants in the early spring. When we have our occasional sunny days, drier soils warm more quickly. Then plant early cool weather crop seed under cover. Asian greens, lettuce, broccoli raab and arugula germinate well undercover in March. If it is sunny, don’t forget to crack open the cold frame or open an end or two of your hoop house. 
• Start your earliest broccoli, cabbage, cauliflower indoors. Or buy starts to get an even earlier crop than my first crop of seedlings. May is a better time to plant some seed directly out in the garden.
• Get your tomato and pepper seedlings planted from seed as early as February through March. Then by April these can be transplanted into larger pots.
• Cold harden tender indoor-grown transplants in late April and put them in the ground under plastic in May until the nights get into the 60s in early June.

Carol Savonen is a naturalist and writer. She is an associate professor emeritus at OSU and tends a large garden in the Coast Range Hills west of Philomath with her husband and dogs. She can be reached at Carol.Savonen@oregonstate.edu or c/o: EESC, 422 Kerr Admin. Bldg., OSU, Corvallis, OR 97331.