The part of the plant to eat for broccoli is the flower, for kohlrabi is the stem, and for brussels sprout is the leaf. I grow all of these in the hardiness zone five of the Midwest.
The first week of April is the best planting time for all three vegetables in my garden. I either purchase the seedlings or grow them from seeds inside. Also I have started the seeds outside around the first of May. When I start the seed in the garden I make a hole the same size as the container used for starting the seed inside and add the same starting medium. Also I use an insecticide dust on the plants after they come above the ground. This keeps insects and other animals away from the seedling as it develops into a 4 to 5 inch plant.
Broccoli can be eaten during first week of June, with the help of the weather. Broccoli can have a bad taste if the large stem is eaten or after the yellow flowers have started from the buds. The groups of buds is the part ate. Typically the first and largest grouping of the buds tastes the best. After the initial harvest the plants grow several smaller groups of buds which are also ate. Around the middle of July the smaller buds loss the original taste. So I remove the plant and put winter squash seeds in its place.
Kohlrabi is ready to eat from the first day of summer to the 4th of July. The ones in my garden taste best went they’re about 1 and 1/2 inch in diameter. Any larger the stem will become hollow and crack inside. I peel off 1/4 inch of the outside to reach the better taste of the inside. After harvest I remove the cut stem and its roots to plant winter squash seed.
Brussels sprouts can appear to stop growing, during the warmer days of summer. But they should be left in the ground, because they start the growth again as the days get cooler. Brussels sprouts need one or two freezes to get their good taste. For the temperature to reach 25 degrees is better than falling just below freezing. For my growing area it might be necessary to wait until middle of October to get the good taste. When harvesting, I remove the bundles of Brussels Sprouts leaves themselves. By not cutting off the stem, new leaf bundles will grow until a hard freeze ends the season.
All three vegetables grown in my garden are rather small. My Kohlrabi grows too around 1 and 1/2 inches in diameter and my Brussels sprouts leaf bundles reach less than 1 inch in diameter. The first time I ever saw the larger ones pictured in catalogs, I was in the hardness zone 4 of Wisconsin. One Saturday morning, at a farmers market, I purchased 4 inch diameter Kohlrabi and 2 inch diameter Brussels sprouts.