\documentclass[9pt,openany]{memoir}

%% Generic packages
\usepackage[utf8]{inputenc}
\usepackage{lipsum}
\usepackage{ninecolors}

%%% Page layout (half letter) %% Specific to Memoir class
% https://tex.stackexchange.com/a/758505/245702
\setstocksize{222.3mm}{142.9mm}
\settrimmedsize{222.3mm}{142.9mm}{*}
%\settrims{3.2mm}{3.2mm}
\setlrmarginsandblock{15mm}{15mm}{*}
\setulmarginsandblock{25mm}{25mm}{*}

%%% Chapter titles %% Specific to Memoir class
\makechapterstyle{kchap}{
\renewcommand*{\chapterheadstart}{\vspace*{\beforechapskip}}
\renewcommand*{\printchaptername}{}
\renewcommand*{\chapternamenum}{\space}
\renewcommand*{\printchapternum}{\hfill\chapnumfont Chapter \thechapter}
\renewcommand*{\afterchapternum}{\par\nobreak\vskip \midchapskip\color{black}\hrule\vspace{5mm}}
\renewcommand*{\printchapternonum}{}
%\renewcommand*{\printchaptertitle}[1]{\chaptitlefont #1}
\renewcommand*{\afterchaptertitle}{\par\nobreak\vskip \afterchapskip}
\renewcommand*{\chapnamefont}{\normalfont\normalsize}
\renewcommand*{\chapnumfont}{\normalfont\large\color{green4}\itshape}
\renewcommand*{\chaptitlefont}{\normalfont\Huge\scshape}
\setlength{\beforechapskip}{0mm}
\setlength{\midchapskip}{2mm}
\setlength{\afterchapskip}{10mm}
}

%%% Page styles %% Specific to Memoir class
%\newcommand{\kinfo}{{\footnotesize Chapter \thechapter{} • \S{} \rightmark{}}}
\newcommand{\kinfo}{{\footnotesize Chapter \thechapter: \leftmark{} • \rightmark}}
\makepagestyle{kpage}
\makeevenhead{kpage}{\textsc{Walking}}{}{\kinfo}
\makeoddhead{kpage}{\kinfo}{}{\textsc{Walking}}
\makeevenfoot{kpage}{}{\thepage}{}
\makeoddfoot{kpage}{}{\thepage}{}
\makeheadrule{kpage}{\textwidth}{0.1mm}
\makefootrule{kpage}{\textwidth}{0.1mm}{2mm}
\makepsmarks{kpage}{
	\nouppercaseheads
	\createmark {chapter}{left}{nonumber}{}{}
	\createmark {section}{right}{nonumber}{}{}
}
\makeatother

%%% required if you have changed the settings! %% Specific to Memoir class
\checkandfixthelayout

%%% Multicolumns
\usepackage{multicol}
\setlength{\columnseprule}{1pt}
\def\columnseprulecolor{\color{brown9}}
\setlength{\columnsep}{5mm}

%%% Fonts
\usepackage{mathptmx}
%\usepackage{newtxtext,newtxmath}


%%% Get it started
\checkandfixthelayout
\begin{document}

\pagestyle{kpage}
\chapterstyle{kchap}


\chapter{Herbaceous Plants}

\begin{multicols}{2}
\noindent{}Thus, it begins.

\lipsum[1-10]
\section{Gentian}
\noindent{}This is the section title: ``''.

\lipsum[11-15]
\section{Bluebonnet}
\lipsum[16-20]
\end{multicols}


\chapter{Non-Seed Bearing}

\begin{multicols}{2}
\noindent{}\lipsum[21-25]
\section{Ferns}
\lipsum[26-30]
\section{Mosses}
\lipsum[31-40]
\end{multicols}


\chapter{Monocots}

\begin{multicols}{2}
\lipsum[41-42]
\section{Grasses}
\lipsum[43-45]
\end{multicols}

\end{document}