Are mustard trees and mustard weeds the same thing?

The mustard seed is unusually small and yet grown to a great size.

The idea seems to be that the tiny beginning of the church will eventually culminate in great growth.

Herbs are garden plants or vegetables. However, such numerical growth will come to harbor the birds (evil ones).

The parable accordingly foreshadows the growth of the church into a world power. However, outward growth is not always a true picture of spiritual depth.

"Kingdom of heaven" is the spiritual form of the kingdom in the church.

"Leaven" is a lump of old dough in a state of fermentation, which makes the bread dough rise.

It is virtually always used as a symbol of evil (Matt. 16:6--12; Mark 8:15; Gal.5:9).

:"Three measures of meal," a common baking quantity (Gen.18:6(, equivalent to one-and-a-half gallons.

The leaven is not just false profession of unsaved church members but false doctrine that they will attempt to bring into the church.

It's marijuana

Well...there are different types (species) of the mustard plant.
https://en.wikipedia.org/wiki/Mustard_plant
Frankly, I'm not sure what you mean by "mustard weed", but I'm guessing that it is, in fact, a species of mustard plant
http://www.ipm.ucdavis.edu/PMG/WEEDS/mustards.html
Almost certainly *not* the same species to which Jesus was referring (a domesticated species).

P.S. This article addresses the Biblical "bird-supporting" mustard plant:
http://ww2.odu.edu/~lmusselm/plant/bible/mustard.php

No. Salvadora persica is the mustard tree. Mustard weeds are from the genera Brassica and Sinapis, not Salvadora

Jesus explained that the mustard seed was very small yet grew into a large tree. That would leave out the weeds.