Ask a local Master Gardener about transplanting seedlings

  • surachetkhamsuk

For the Gazette
Published: 3/28/2019 4:55:30 PM

Q:  How do I know when I am ready to transplant my vegetable seedlings to my garden outside? My early ones seem about ready. —C. H. Northampton

A:  Timing when to transplant seedlings is always a bit tricky, C.H., as it is not an exact science but rather varies with the weather and soil conditions. Our average last frost date is around May 15. Your seed packet should have some helpful planting information on it related to that date. Also, before transplanting to the garden, make sure these things happen: you see the first set of true leaves and you harden off the seedlings.

A “true” leaf is one that can perform photosynthesis, the process plants use to make their own food. When a seedling sprouts, the first leaves that appear are not “true” leaves, but rather “cotyledons” or seed leaves. They sometimes still have the seed coat on their tip as they grow upwards. If the plant has hypogeal germination, as with peas, the seed leaves stay below the soil and you likely will not see them except potentially during transplanting.

Cotyledons provide the baby seedling with food until the true leaves rise up above them and start making their own food for the plant through photosynthesis. True leaves resemble the look of the adult leaves whereas cotyledons are simple and nondescript. You want to see 3 – 4 true leaves before transplanting. This ensures the plant can feed itself after it is transplanted.

Once your seedlings have these leaves, it is time to start hardening them off. Hardening off is a process that starts acclimating plants to the more rugged outdoor climate after being pampered inside. You can accomplish this task a couple of different ways.

One way is by moving the seedlings outdoors to a partially shaded area for a few hours a day then bringing them back indoors afterward, gradually increasing their amount of time outside and sun exposure by 1 – 2 hours each day. Do this for 7 to 10 days and they should be ready to move into the garden, assuming the soil is dry enough. If a few inches of soil sticks to the bottom of your shoe when you step in the garden, it is too wet for transplanting.

Another way to harden off seedlings is by placing them in a cold frame for 7 – 10 days before planting in the garden. A cold frame is a simple, 4-sided structure built low to the ground, often made of boards or loose concrete blocks, with a moveable, transparent top for letting in sun. There are a wide variety of commercially available ones or you can easily build one yourself. You open and shut the top by varying amounts to help regulate the inside temperature on sunny days. Make sure you have a thermometer in the frame to help with this temperature control. You do not want to accidentally cook your plants!   

Once you have accomplished these tasks, and the garden soil is workable, you should be ready to go. Good luck with your gardening, C.H., and thanks for asking a local Master Gardener.

Have a gardening dilemma? Please send questions, along with your name/initials and community, to the Western Massachusetts Master Gardener Association at One question will be selected and answered per week.

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