I was wandering if symbols like these (used in garden/landscape design) can be found as a scalable font:
The garden design plans that would be made using such font would be like this:
Can such symbols be found in a True Type font?
The reason to do a font like collection of symbols is if your system is somehow limited in capability. Such fonts do not have any benefit in a system designed to drop in such symbols.
Most software geared towards landscape design have a library feature so most likely most professionals would have no use for a font. Therefore you may or may not find very many such fonts
You can always make your own font. Export your drawings as svg or eps and open them in a font editor. Like say FontForge, which is free. All you need is to either draw a few symbols, or import them from some suitable clipart (like this).
Image 1: A quickly made font with 3 symbols
As some previous answers are guessing that you are not running the latest Adobe Package, here are just some ideas to help you search further: Many moons ago, Corel used to provide such fonts, but called them "symbols". Technically they are .ttf files, just like you requested.
I believe you will struggle, finding those fonts on their own, but you might be finding old Corel Draw packages (say Corel Draw 6 upward and at least until version 8). Those are now dinosaurs, but came with nicely printed massive catalogs on paper and stacks of DVDs. So no, I would never sell my own, but try Ebay in your country.
In a Corel printed clipart catalog, you need to find a separate section called symbols. For example I found some "from above plant symbols" in the sections called "Home Planning 1" and "Office Planning". The respective fonts are called homeplan.ttf and offcplan.ttf - maybe you get lucky with a web-search.
Still, I would agree with the previous answers that you are making things hard on yourself. At least consider a free layout tool like Scribus, where you can have tools for using glpyhs from fonts as symbols but also many more tools for making plans and importing vectors directly etc.
I personally find Inkscape rather limited when working with text (when coming from the font perspective), but if you are making "pure garden plans" like in your examples above, Inkscape might be a better tool even than Scribus. You can later include a plan (as .eps for example) in a Scribus-document if you want to add a title and description and more meta-data in text-form.
I bet you will see some comments below, which will confirm this to you, or try to convince you towards Adobe or whatever. Good luck with those fonts, if they are working for you. How did you make those examples anyway, they look nice to me?