Mesclun (pronounced mes-CLOON), the Nicoise dialect word for a mixture of tender, young salad greens, is the ideal crop to start in a cold frame around the time of the first frost. The cold frame will keep the greens harvestable well into winter. In the protected environment of the cold frame, the seeds of lettuce, greens and herbs will germinate quickly and be ready for picking in a matter of weeks.
Many mail-order catalogs offer ready-made cold frames. If you don't already have one, you can construct a cold frame quickly using acrylic sheets, plastic film or an old windowpane for the cover, and scrap lumber or bales of hay for the base. To capture maximum heat from the sun's rays, slope the lid toward the south by tapering the wooden sides or sloping the bed beneath the bales. The cold frame should be located where is is protected from the wind.
1. Prepare the soil
With a spading fork, loosen the soil under the cold frame and remove any rocks. Add enough mature compost to make a layer three inches deep. If a soil test has indicated that your pH is not between 6.5 and 6.8, you may need to adjust it with finely ground limestone or ground sulfur. Incorporate these amendments evenly into the soil and rake it smooth.
2. Sow the seeds
I prefer to buy individual packages of a variety of greens and to sow the seeds in short, closely spaced rows rather than simply boradcasting everything. This method gives me more control over the spacing of the rows and lets me segregate the stronger-growing greens where they won't crowd out the smaller plants. Make shallow furrows four to five inches apart across the cold frame. Sow seeds about an inch apart in each furrow. Labeling each row will help you identify the greens.
Cover the seeds with 1/4 inch of soil or damp, finely milled sphagnum moss. Many gardeners have found that the latter helps prevent the seedlings from damping off.
3. Care for the seedlings
Water frequently, keeping the soil moist but not soggy, until the seeds germinate (usually in one week or less). Continue watering the plants as needed. As the weather cools in late fall, little or no irrigation will be required.
After young plants have developed two or three sets of true leaves, give them a light dose of a one-quarter-strength, complete, soluble fertilizer to encourage faster growth. Don't over do it; too much fertilizer will encourage soft growth, which is more susceptible to frost damage.
4. Vent the cold frame
Use a thermometer to monitor temperatures inside the cold frame. On sunny days when it reaches 65 degrees F, you will need to vent your cold frame to prevent hot air from damaging your plants. A stick makes a convenient prop. For more permanent cold frames, you can install a temperature-sensitive automatic venting arm, available through many mail-order suppliers, to lift and lower the lid. Be sure to close the lid in the late afternoon to conserve heat.
Start harvesting your mesclun when the biggest plants reach three to four inches tall. Begin with the larger leaves, cutting them an inch above the crown with scissors or a sharp knife. If the plants are very crowded, you can also harvest by thinning out entire plants. Continue picking from different parts of the bed every few days. As you combine the various greens, each harvest and salad will be unique. Depending on your climate and the varieties you have planted, the plants may regrow and give further harvests.
When really cold weather arrives, plan to harvest when the temperature in the cold frame is above freezing. Greens that might survive a freeze will be limp and unappetizing if picked while frozen.