Ask Mr. Smarty Plants

We will have to say we have never been presented with exactly this set of symptoms from a Sophora secundiflora (Texas mountain-laurel) before, and we sure don't know what is causing them. Let's begin with the basics. According to this USDA Plant Profile, the Texas Mountain Laurel does not grow in North Central Texas, including Dallas County. Obviously, you can buy anything and plant it anywhere, but that doesn't mean it's going to be happy there. It is a lush, evergreen plant with its fragrant purple flowers that seems like it should be a tropical plant from a rain forest. Not. This is a desert plant, accustomed to surviving, blooming and propagating itself without human intervention in sometimes harsh conditions. From our Native Plant Database, here is the Soil Description preferred by this plant.

"Brushy slopes; open plains. Common in limestone soils. Well-drained sand, loam, clay, caliche, limestone."

The same source indicates it likes a highly alkaline soil, dry conditions and sun or part shade. It absolutely requires good drainage in the soil. Frankly, we think you are loving your Mountain Laurel to death. It grew so well, it can't support itself. It is ordinarily a slow-growing plant, partly because it must adapt to desert conditions just to survive. 

Here's our prescription, we have no idea if this will work, and if the tree dies, it may be because it is already dying. First, and foremost, NO FERTILIZER! ever. Trees planted where they belong subsist on the soils they are acclimated to by millions of years of experience, and this is a plant that particularly does not like fertilizer. Second, cut out the watering. If your tree is in range of an automatic sprinkler, try to reset the sprayers or in some other way lower the amount of water it is receiving. Now, check the drainage of the soil. Is the tree in a "tree well?" Bad news. Water will collect in that, whether or not it drains well. Since most of your area has heavy clay soils, that tree's roots may be drowning. Your area has had more rain than normal the past few months, which isn't helping. Correcting drainage for a tree already in the ground for 3 years is almost impossible. Again, cut down on the water that gets to it, keep the roots mulched with a good shredded hardwood mulch to help soak up some of that water. As that mulch decomposes, it should begin to improve the texture and drainage of the soil. Use continued applications as the level of the mulch lowers to very gradually raise the soil level.

The only other suggestion we can make, because that tree should NOT be lying on the ground, is to trim some of the upper foliage. Treat it like you would transplant shock (which still could be a contributing cause even after 3 years) and trim off about 1/3 of the upper portion of the plant. This will take some of the burden away from the roots of supplying water and nutrients to the upper leaves. If you still want to stake the plant, here are some instructions extracted from a This Old House website:

"Once you straighten a tree, it's a good idea to stake it. Drive a couple of wood stakes on the side of each tree. Then run wire or rope from the stakes and wrap it loosely around the trunk about two-thirds of the way up. To protect the bark against chafe, cover the wire or rope with scrap pieces of garden hose. You shouldn't let your trees become too attached to their stakes. Unstaked trees develop stronger root systems than those with artificial supports. So do your evergreens a favor and take the stakes out next spring."

As for training the tree to be multi-trunk, you can see from the pictures below that they naturally put out more than one trunk. We think your tree will do that once it becomes stronger and has the energy for something more than survival. You can train a tree to have a single leader by pruning away secondary trunks, so the only way to "train" it to be multi-trunk is to give some more trunks a chance to emerge. 

From our Native Plant Image Gallery:

Sophora secundiflora

Sophora secundiflora

Sophora secundiflora

Sophora secundiflora