The other scholarly publication in our field is the Journal of Garden History, a publication of the Garden History Society.
"Garden History is one of the foremost journals in its field worldwide and remains the journal in which to be published for those dedicated to garden history scholarship. It has a wide academic and professional readership, and from the earliest issue to the present is an enormously important and relevant source of information, providing vital support to the society's promotion of the study of garden history, landscape gardening and horticulture."
The GHS is based in England, but wherever you live you, you really should belong if you're seriously interested in the subject. Membership is just ₤43 a year, which entitles you to the journal and attendance at GHS events if you happen to be in England. But just receiving the journal is well worth the price. No online application, but it's not so hard to actually mail something, is it? As a member, you can be in touch with a group of people who are passionate and knowledgeable about garden conservation and historical study. Peruse their website, where there are many helpful links and a suggested reading list.
Jof GH is catalogued by the archive of scholarly journals, JSTOR (Studies.. is not) , so if you have access to the beloved JSTOR (http://www.jstor.org/) through a university or your big-city library, you can search the articles and print them out for leisurely perusal. I myself was published last year in the Journal, and hope to be this year as well.