Helping seedlings cope with planting

The broccoli and onion seedlings arrived from a Southern nursery two weeks ago. And because they were field-grown and consequently hardy, they could be planted in our garden without any concern that they wouldn't survive.

In fact, the plants were tested by high winds and a few snowflakes within days of going into the garden, without any ill effects.

The same cannot be said for my home-grown seedlings, which, since germination , have enjoyed a protective indoor environment under artificial light and heat. They have never had to withstand the beating of rain, buffeting of wind, or fluctuating temperatures, and, as a result, will have to be appropriately ''hardened'' before going outside.

I bring up this point because a new-to-gardening friend of mine, delighted with his home-grown seedlings, was about to commit them to the great outdoors without any preparation whatever. My friend had not heard of the need to ''harden off'' seedlings. ''Hardenening off'' simply means toughening up the seedlings so they can withstand the more rugged outdoor climate. The toughening-up process isn't difficult as long as you:

* Lower the temperature at night. Bring down the temperature in the room where the seedlings are growing by about 10 degrees, and even more if they are the frost-hardy types, such as cabbage. Tomatoes, peppers, and other frost-tender plants should not be overly stressed in this respect, but can be hardened at plus or minus 60 degrees F.

* Reduce water. Allow the soil to dry somewhat between waterings, even to the extent of allowing the seedlings to droop a little. For some reason, the plants toughen up rapidly when mildly stressed in this way.

* Do not add fertilizer. If the plants have used up most of the nutrients in the starter cups or flats, even to the extent of turning a little yellow, so much the better. Fresh, succulent tissue is less able to withstand heat, cold, and wind than the more fibrous older tissue. Feed the seedlings when they go into the garden and they will respond quickly and vigorously.

* Expose the seedlings for a few hours each day to the direct rays of the sun , increasing the time each day. This can be done indoors in a sunny window if your wish. If, of course, you have raised your seedlings in a window or greenhouse, this treatment will not be necessary.

* Avoid windburn. Finally, set the seedlings outdoors for a few hours each day, but only in a sheltered spot where wind will not burn them. Some air movement will inevitably get to your plants - and this will steadily harden them. In this respect, a partly open cold frame makes an ideal area for hardening the plants.

Whatever you do, do not cut back on the light a plant receives during the hardening treatment. Darkness does nothing to toughen a plant; it merely stunts its growth.

The window area of a garden shed in my home makes an ideal hardening-off location for plants. It is exposed to morning sun but sheltered from the afternoon rays. The shed itself is protected from the prevailing winds by the house. At the same time, the large rear window of the shed is missing, so that some air movement does take place through that window and the open door on one side of the shed.

If you can duplicate that sort of environment, you should have no trouble toughening up your seedlings.

I have also found that plastic milk jugs with the bottoms cut out provide an almost ideal environment for hardening off plants, provided they have had a week of reduced waterings and lower night temperatures before going out.

Place the jug firmly over the young seedling and anchor it well. Also, mound a little soil around the base of the jug to prevent any convection currents starting up. The milky color of the jug allows all the necessary light to get through, but tempers the intense heat of the sun just a little. At the same time , air entering through the mouth of the jug helps to accustom the plant to air movement.

Most nurseries harden off their flats of plants before offering them for sale. If the nurseries do not do it, they won't get your business next year.

Another option is to buy bare-root started plants that have been field-grown by a nursery in the South. While they may look a little beaten and droopy when they arrive at your doorstep, the roots are in good shape and the plants take only a few days to revive in their new environment.

Further, the plants are tough. After all, the great outdoors is all they have ever known.