Till a 6-inch layer of compost tnto your soil several weeks before planting to greatly improve its fertility. Or, build a raised bed and fill it with a combination of purchased topsoil and compost. Mulch plants with organic matter, such as compost, shredded bark, grass clippings, or straw. Continue to add lots of organic matter each year, which over time will not only improve your soil's fertility and drainage, but will also increase its ability to retain moisture and nutrients. Organic matter also provides food for earthworms and microorganisms that help build the soil.
You may be removing too much foliage when you harvest. It's always best to harvest sparingly the first year, to give the plant time to become established. Here are some general guidelines to harvesting herbs: Begin harvesting the herb when the plant has enough foliage to maintain growth. Up to 75% of the current season's growth can be harvested at one time. Harvest early in the morning, after the dew dries, but before the heat of the day. Harvest herbs before flowering, otherwise, leaf production declines. Herb leaves have their most intense oil concentration and flavor when harvested after flower buds appear but before they open. Herb flowers harvested to dry for craft purposes should be picked just before they are fully opened. Annual herbs can be harvested until frost. Perennial herbs can be clipped until late August. Stop harvesting about one month before the frost date. Late pruning could encourage tender growth that cannot harden-off before winter. Harvest tarragon or lavender flowers in early summer and then shear the plants to half their height to encourage a second flowering period in the fall. Hope your next attempt is more successful!
With a cold frame, you can grow many different cool-season vegetables--plus get a head start on early spring crops. Concentrate on root crops and green leafy veggies during the fall and winter months. Lettuce, cabbage, broccoli, carrots, turnips and chard are good choices for the low light and cooler temperatures of fall and winter. You may want to do a planting right in the garden in late August, and save the cold frame for a second planting in a month or so, for an extended harvest. You can cover the garden plants with row covers during chilly nights to help extend the season up until a hard freeze.