Rhizobium spp. are bacteria that lives in some soils. They work in association with the bluebonnet’s root system to fix nitrogen from the air. Rhizobium spp. may or may not be present in your soil, so it is recommended that you use it at the time of planting to be sure it is there. Bluebonnets growing without Rhizobium tend to be small and weak, and do not produce as many flowers.
Bluebonnets do not need Rhizobium to germinate or sprout; however, the key word is germinate. Bluebonnets growing in native soil do need Rhizobium after their first root and leaves appear, and Rhizobium becomes important as the seedling starts to grow. The root finds the Rhizobium, if it is present, and then fixes nitrogen from the air to help the plant grow. In return Rhizobium obtains nutrients from the host plant.
Rhizobium already may be present in your soil. If you only planted a small area, you might want to get a packet of Rhizobium and mix it with water and apply it to the soil immediately around your bluebonnets. If you planted a large area, it is probably not worth applying Rhizobium.
Right before planting your seeds, moisten them with water and then sprinkle with the Rhizobium, which looks like a black powder. Some people use milk instead of water because it is said to be stickier. Rhizobium will stick to the seeds and stay with the plant after the seeds germinate. Since Rhizobium is alive, be sure to use fresh Rhizobium and store it in a cool, dry place until use.
The bluebonnet’s hard seed coat is a wonderful mechanism for survival. Some bluebonnets will germinate right after planting, while others will lie dormant until the seed coat is worn down enough to let water in and allow germination. To plant bluebonnet seeds Mother Nature’s way, prepare your soil, use your Rhizobium, and plant your seeds. Make sure the seeds come in contact with the soil. You can choose to water them or let nature take it’s course. During a dry winter, it is a good idea to water your bed occasionally, but they don’t like to be kept wet.
You may have a better chance of quick seed germination if you scarify the small seeds. Scarify means to scar the seed coat, which lets water in and stimulates germination. You can put bluebonnet seeds in your freezer overnight, then boil some water and let the seeds soak in the hot water for an hour or two. The freeze/thaw action breaks the seed coat and the soaking allows water necessary for germination to seep in. Pour the water off, sprinkle the seeds with Rhizobium, and plant the seeds. DO NOT let your students do the bluebonnet stomp on the new seeds because these seeds are plump and juicy; they are extremely vulnerable. If you decide to scarify your seeds this way, you owe it to the seeds to keep them evenly moist during their seedling stage.
For more information, read the Wildflower Center’s Growing Bluebonnets How To Article or search Mr. Smarty Plants for Frequently Asked Bluebonnet Questions.