In order to maximize the yield out of my garden I plant my brassicas about 30 inches apart in my four foot wide raised beds. In between each of the brassicas, I plant a lettuce seedling which will be ready to harvest about 30 days after planting. After planting my lettuce and brassica seedlings, I water the seedlings in with a half strength solution of water soluble fertilizer. You can use either an inorganic or organic fertilizer, but if you use an organic fertilizer, make sure that most of the nitrogen is in a water soluble form. For example, fish emulsion generally has an NPK of 5-1-1. According to the label on the back of my bottle, most of the 5% N in the bottle is in the form of water soluble nitrogen and will be immediately available for uptake by the plants roots.
This method of interplanting brassicas and lettuce has served me well over the years, since any lettuce not harvested will be shaded by the larger brassica. In most years, I just pick the outer leaves of the lettuce, letting the center of the plant grow until the summer turns it bitter. This diagram shows my planting method.
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This photo show the planting as of April 4.
The row cover on the left of the photo is placed over the bed with the wire hoops making a mini hoop house. My row cover is really insect barrier protecting my brassicas from the imported cabbage worm or the little white butterflies with black spots on their wings. (see http://extension.umd.edu/growit/insects/imported-cabbageworm. On April 28, I thought I would open the row cover and remove any weeds that had sprouted over the last month. There were plenty of weeds, but as you can see from the photo below, the brassicas and lettuce have made great growth. I'm definitely ready to add some fresh buttercrunch lettuce to my spring spinach salads.
One final word on brassicas. These are cool weather plants and need to be grown quickly in the spring. For that reason, I grow varieties that have the shortest "days to maturity" from time of transplanting. The broccoli I grow is "Packman" which reaches maturity in 55 days. My cauliflower is "Snow Crown" which is 60 days to maturity and my early cabbage is "Golden Acre", also 60 days to maturity. I also transplanted some "Early White Vienna" kohlrabi which yields nice swollen stem by mid May. My fall brassica tend to have longer maturity times and the broccoli produces many side shoots. Once again this fall I'll be growing romanesco cauliflower called "Veronica" (it has a great nutty flavor). It takes 78 days to reach maturity and I usually plant it in the first week of August. This gives me a green cauliflower for the table about the end of October (78 days plus the short day factor of 14 days).