I have known some architects that practice landscape architecture. Theres a larger local la firm in our area, their local office is led by a trained architect, though I dont think he has practiced building design in several decades. 

I dont think having an architecture background will hold you back. There will be a whole bunch of new things to learn--particularly planting design, grading and drainage, irrigation, different material palette, etc, but compared to architecture I think landscape is relatively simple, though deceptively so. 

Its a looser medium than architecture, sometimes with little or no specific programming. Spacemaking is a more esoteric exercise and often involves more problem finding and solving than design for designs sake. 

I'd be happy to discuss more if you're still following this post..

I've worked with a lot of architects doing LA.  It'll depend a lot on the type of work - the spatial design translates fairly easily and the hardscape as well.  Grading seems to be a challenge for many, soils and plants are something you'll have to have a strong interest in to get a good handle on.  In general the rigor of architecture school makes the la degree unnecessary (a lot of high caliber firms prefer it as many la programs are not as cut throat so deadlines are easier met by those that thrived in an arch program).  Architects can be trained to do both, but it's a different mindset.

I got both in school (dabbled in arch while in engineering undergrad, then mla/march).  The field is much more laid back, and tends to take a larger look at things.  Fewer peacocks and a vision for growth of a project over a few decades (excepting some projects like plazas) tend to keep the ego in check for the most part. Mostly you'll work at shaping space in a much more subtle way which takes a lot of getting used to.

Yep - studied and got a masters of architecture, but realized how much more interested I was in the public realm and focused my masters projects towards that.

I only pursued work at landscape / urban design firms since graduating in 2011. It hasn't been a problem to advance without an MLA but for myself I want to take a course in botany and soil to bolster what I've been learning on the job. I started volunteering at an urban farm to get more experience. 

I'm told that working at a tree nursery is a great way to get a comprehensive understanding of local plants and their care needs. So if you want to pursue additional education, look for experience-based/hands on stuff rather than a formalized degree. I would imagine that a lot of the same design principles taught in architecture are mirrored in landscape.

In the province I work in, they have different levels of membership within the landscape association. You can join as a professional with relevant work experience and work towards getting accredited that way; it's not obligatory to have a degree. 

Yes- I made the switch by doing a masters in landscape after my BArch. I felt the fields were too divided. They still are. First I was on the side of the architects asking landscape architects to fill in the space around the building. Now I'm on the other side looking at architectural drawings and shaking my head at poor design decisions while being asked to design the space around them at a landscape architecture office.

We need more people trained in and with practical experience in both fields, and more offices that work on architecture and landscape with one vision and one team.

I do think the extra schooling helped. With only architecture training you sometimes miss the background and references required to solve complex landscape problems. The field is so large...between a garden and a regional green network the challenges are very different. Architects can just about handle paved urban space (though most tend to overdo the design) but are not good with softer more natural approaches. The skills from an MArch do generally put you ahead of MLA students as one posters said, because you were pushed way harder in your education.

Keep in mind a building is "pristine" on the day it is delivered to the client, photographed and then the architects more or less walk away. Landscape designs take years to come to their peak, and the maintenance of the design after construction and over those years is half the battle. I have seen planting schemes put in one year at great expense and then systematically destroyed over the next 2 seasons by inept maintenance crews only to be left with the same sad lawn scape that was there before the designers started.