Fertilizer:
In the first two years you DO NOT want to add any extra nitrogen to get the runners to fill in the bed. Most cranberry plants will grow very vigorously without added nitrogen. This seems to be one of the biggest problems gardener's have told us...that the plants are too vigorous. Once the bed is filled in with runners, you will be able to tell if the planting needs more nitrogen by the number and lenth of new shoots. if there is alot of runners, do not fertilize. If it looks like the plants are struggling to fill in the bed, We suggest applying a fish emulsion fertilizer (2-4-2) at a rate of ½ gallon once at the beginnning of growth in the spring and once more once the berries start forming. Again, be careful with nitrogen!
It bears repeating here that flowers and fruits form on the uprights. Uprights form at the nodes of the runners. Once the bed is filled in with runners, which takes about two years, then the plants will start to form uprights. You need to withhold nitrogen, and IF the runners are inhibited from growing strongly they will produce uprights. Hence, the less the fertilizer after the first two years,the less the runner growth, and the more the uprights will form. Too much fertilizer, and you will reduce the number of uprights that are formed and you will not get berries.
One of the most comments we get from our customers is the small amount of fruit they sometimes get. Once we see the pictures, we can see that the planting has been fertilized too much, and runners are everywhere. The best way to see how much fertilizer you need is to test the soil every other year by contacting your Extension Office and getting a soil testing kit. These tests can give you a better indication of how much fertilizer to add so that you do not over fertilize. Use your test results and compare them to the commercial grower information found in Washington, Oregon, Wiscosin, New Jersey or Massachusetts.