Steve asks:
>Can hybrids of Ludwigia be formed by cross-
pollination or are they done by grafting or other methods?

Hybrids are formed when the male parent of a plant (or animal) is of a
different species than the female parent.
eg: Ludwigia repens x L. arcuata  (note: female x male)
L. arcuata x L. repens is a different  plant.

BTW, grafting leaves the genetic make up alone. You simply have two or more 
plants sharing common structures (eg. two apple tree branches from to different
species could share the stem and root system of yet a third apple species.)

>What are the identifying
features of Ludwigia glandulosa?
As per Christel Kasselmann's Aquarienpflanzen, L. glandulosa's identifying
structures are a woody, squarish stem (i.e. it has "edges" as opposed to being
round). Leaves are olive-green to wine-red on top and wine-red on the bottom.
It will lose its lower leaves in a poor substrate, low light, and temperature
over 25 deg. C. (78 deg F). Also, it has NO petals.
The flower looks like that of L.repens without the yellow petals.

Since you have yellow petals on your flowers, I doubt that you have
L. glandolosa.
I believe that you have one of the hybrids L.repens x L.arcuata, L. palustris
x L. repens or even L. palustris x L. arcuata.
Also, don't forget that the different Ludwigias come in a number of varieties
such as red and green forms of L. repens.