I strongly suggest you give up the idea of counting plants as individual blobs, and just go with "area fraction of green" as your metric if you're aiming to determine growth of the plants.
If you're trying to determine germination rate, like how many plants per square meter per known weight of seeds (and hence number of seeds) spread over that square meter, then you should spend the time to make up a calibration table. To do this you plant a number of seeds well separated from each other. Then count how count how many germinated and compute the green area fraction. Then wait a few days and compute the area fraction again, and so on until the plants are as old as you ever expect to need to run this analysis. Now divide the area fraction by the known number of plants in the image to get the area fraction per plant as a function of age. After this you will have a calibration table that is a two column matrix where age is the first column, and the area fraction for a single plant is the second column.
Now, to measure a test image, you measure the area fraction. Then you go to the row that represents the known age of the plants (measured in the same way you did for constructing the calibration), and look up the area fraction for one plant in your calibration table for that age. You might have to interpolate between ages if the age is not directly in the table. Then you simply divide the area fraction of your image by the area fraction of a single plant to get an estimate of the number of plants in the image.
Note that this or any method will start to break down once the plants overlap a lot (which is undetectable in the image). for example if the image is all green, we have no idea how many plants there are -- could be one, could be hundreds.