Ask Mr. Smarty Plants

There are a number of plants referred to as clovers or having "clover" in their common name. The true clovers are all members of the Trifolium genus, and the Fabaceae or pea family. We found four of these that are native to Washington, including Trifolium fucatum (bull clover),  Trifolium macrocephalum (largehead clover), Trifolium willdenowii (tomcat clover) and Trifolium wormskioldii (cows clover). 

These plants are all legumes, members of the Fabaceae, or pea family, of which there are many genera and species, all sharing the common trait of being able to transfer nitrogen from the air into their roots. When the plant dies, the nitrogen is released into the soil, making it available to other plants. You could say that the clover is "fixing" the problem.This Gardening Know How website Nitrogen Nodules and Nitrogen Fixing Plants explains this better than we can. 

All legumes perform the same function but there are easier ways to get nitrogen into your soil for your plants than planting a variety of pea plants. For instance, if you want your grass to take over the clover, instead of the reverse, fertilize the grass with a higher nitrogen and lower phosphorus fertilizer. If you feel some of your plants are suffering from a lack of nitrogen, fertilizing with a balanced fertilizer should help. You do not want to use a too-high nitrogen content fertilizer for ornamentals, as it will produce lots of green leaves, which is what you want in a grass, but at the expense of the amount of flowering.

Plants require a mix of nutrients to remain healthy. Nutrients that are needed in relatively large amounts are called macronutrients, and include nitrogen, potassium, phosphorus, calcium, sulfur and magnesium. These nutrients should be available from the soil, but the best way to make them more accessible to the plant roots is to keep the soil aerated. Adding compost or other organic material to the soil, mulching the surface, permitting decomposition to add still more nutrients, and making sure sufficient water is being provided will all contribute. 

If you're really serious about planting nitrogen-fixing plants, consider the Lupine, genus lupinus.  Our beloved Texas Bluebonnet is Lupinus texensis (Texas bluebonnet), but this would probably not prosper in Washington. However, you have some lupines that are native to Washington, including Lupinus bicolor (miniature lupine), Lupinus polyphyllus (bigleaf lupine) and Lupinus sericeus (silky lupine).

Trifolium fucatum

Trifolium willdenowii

Lupinus texensis

Lupinus bicolor

Lupinus polyphyllus

Lupinus sericeus