An effective way of controlling aphids is to spray the tree with a fairly strong stream of water. It will knock many of the aphids off, breaking their mouth parts in the process, and causing them to starve. You will need to do this consistently at 3-5 day intervals to prevent the next generation of aphids from getting established. Usually a two weeks of diligence will reduce the numbers to the extend that ladybugs and other beneficials can get to work and take over.

A 15 to 20 foot gap is not sufficient for a willow tree. When I planted the Weeping willow in my dads yard, it was a whip cut from a big tree in my uncle's yard. About 20 years later it had a trunk about 12 inches in diameter, and the canopy was at least 30 feet across. We had a tree that was about 10 years old and it was big enough for an adult to climb in. even that your the canopy was at least 10' across. I love the landscaping qualities of willow. the leaves tend to be a lighter green than many other trees, and I like the contrast. I would look for something else. If that gap gets a lot of sun you may consider a crepe myrtle, or an old fashion lilac. The would fill the gap and add color to your back yard. the Old fashion lilac will also provide a nice odor to the yard in the spring.

this is not a trading forum .. but instead of reporting you.. i will just tell you .... they root in a few weeks.. if you have a local one.. just get some 8 inch cuttings ... either stick them straight int he ground.. of in water until you just see roots.. then pot up ... google it.... if you dont believe me .... ken

I think you mean invasive :-) And neither is really considered an invasive species although both will have aggressively water seeking root systems. Plant far away from any septic system or underground sewer or water pipes.