Here's a question we receive every day: Do I need to harden off my plugs? The short answer is yes! Hardened-off plants are healthier, heartier, and produce better stems. Read on for the long answer, and the three exceptions to the hardening-off rule.
When they arrive, your trays have been in transit for two to four days, and were growing in a high tech greenhouse before they shipped. Use a bit of caution when exposing them to real life conditions. We have all heard of hardening off plants before transplanting.
There are three main factors in transitioning a plant from its perfect greenhouse life to actual growing conditions. Sunshine, wind/dry air, and extreme temperatures are the main concerns in hardening off. Over a few days, expose your plugs to more and more natural light. Start them with a couple hours of morning and evening sun, increasing their exposure each day. By the third or fourth day they should have adjusted their leaves to your sun levels and will be ready for transplant.
During this time they will also have adapted to your humidity levels and will be better able to manage their transpiration of water. Keep them out of strong dry winds during this period. Temperature is not as big of a concern as you may think it is. Most of the plugs we sell are frost hardy, but it is best to keep them above 32F while they are in the plug tray. Roots are more exposed in a tray than in the ground, so prevent freezing of new plugs for best results. Plugs that would survive in the ground may die in a tray, if allowed to freeze. (Tender summer annuals such as zinnia, cosmos, ageratum, gomphrena, celosia etc. should be kept above 55F at all times.) After a few days your plugs will be ready to transplant.
Can I skip hardening off?
There are three ways to skip hardening off.
1. If you are growing in a high or low tunnel you don't need to worry about acclimating your plants. UV rays can't penetrate most tunnel/greenhouse covers and extreme wind and temperature are also moderated, so simply transplant on arrival.
2. If you are planting in the field, simply transplant, immediately irrigate with with a hose or sprinkler, and then cover with remay/frost cloth/agribon. This material increases humidity, buffers the sun's rays, and evens out extremes in temperatures. After a couple of weeks your plants should be growing well, and the fabric can come off.
3. If you are fairly sure you are going to have 2-3 days of cloudy or rainy weather, you can transplant without hardening off. This isn't always reliable as weather changes quickly, but sometimes you can get your plugs in the ground just before the rain sets in, and they will settle in quickly. In the darker shorter days of autumn hardening off can likely be skipped as well if the weather is cool and damp as the sun's rays are less intense.
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