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Yes, Kohlrabi leaves are also edible and very high in nutrients. They can be chopped up and put in salads or cooked like greens. The younger leaves will give you the most flavor.

Your plants have bolted. Once this happens, they become woody and bitter and are no longer good. Here is more information: https://www.gardeningknowhow.com/edible/vegetables/vgen/what-is-bolting-what-it-means-when-a-plant-bolts.htm

This can happen for a few reasons. The first is that they were not planted deep enough. Another reason can be that they were stressed, especially if they were stressed as seedlings. They have a harder time forming a bulb. Too little light can keep them from forming a proper bulb and too little nutrients as well.

Your plants have bolted. Once this happens, they become woody and bitter and are no longer good. Here is more information: https://www.gardeningknowhow.com/edible/vegetables/vgen/what-is-bolting-what-it-means-when-a-plant-bolts.htm This article will help with growing kohlrabi plants: https://www.gardeningknowhow.com/edible/vegetables/kohlrabi/how-to-grow-kohlrabi.htm

This article will help you.
Too warm of weather and other stresses can cause lack of bulbing.

https://www.gardeningknowhow.com/edible/vegetables/kohlrabi/how-to-grow-kohlrabi.htm

These need extremely fertile, moisture-retentive soil, so you may want to add compost, or fertilizers before planting. They also prefer cooler weather, either spring or fall. This article will give you more information on the care of these: https://www.gardeningknowhow.com/edible/vegetables/kohlrabi/how-to-grow-kohlrabi.htm