diff --git a/R/R/prophet.R b/R/R/prophet.R
index c288c23..e9e696a 100644
--- a/R/R/prophet.R
+++ b/R/R/prophet.R
@@ -20,8 +20,8 @@ globalVariables(c(
 #'  also have a column cap that specifies the capacity at each ds. If not
 #'  provided, then the model object will be instantiated but not fit; use
 #'  fit.prophet(m, df) to fit the model.
-#' @param growth String 'linear' or 'logistic' to specify a linear or logistic
-#'  trend.
+#' @param growth String 'linear', 'logistic', or 'flat' to specify a linear, logistic
+#'  or flat trend.
 #' @param changepoints Vector of dates at which to include potential
 #'  changepoints. If not specified, potential changepoints are selected
 #'  automatically.
@@ -154,8 +154,8 @@ prophet <- function(df = NULL,
 #'
 #' @keywords internal
 validate_inputs <- function(m) {
-  if (!(m$growth %in% c('linear', 'logistic'))) {
-    stop("Parameter 'growth' should be 'linear' or 'logistic'.")
+  if (!(m$growth %in% c('linear', 'logistic', 'flat'))) {
+    stop("Parameter 'growth' should be 'linear', 'logistic', or 'flat'.")
   }
   if ((m$changepoint.range < 0) | (m$changepoint.range > 1)) {
     stop("Parameter 'changepoint.range' must be in [0, 1]")
@@ -1053,7 +1053,28 @@ set_auto_seasonalities <- function(m) {
   return(m)
 }
 
-#' Initialize linear growth.
+#' Initialize flat growth.
+#'
+#' Provides a strong initialization for flat growth by setting the
+#' growth to 0 and calculates the offset parameter that pass the 
+#' function through the mean of the the y_scaled values.
+#'
+#' @param df Data frame with columns ds (date), y_scaled (scaled time series),
+#'  and t (scaled time).
+#'
+#' @return A vector (k, m) with the rate (k) and offset (m) of the flat
+#'  growth function.
+#'
+#' @keywords internal
+flat_growth_init <- function(df) {
+  # Initialize the rate
+  k <- 0
+  # And the offset
+  m <- mean(df$y_scaled)
+  return(c(k, m))
+}
+
+#' Initialize constant growth.
 #'
 #' Provides a strong initialization for linear growth by calculating the
 #' growth and offset parameters that pass the function through the first and
@@ -1177,7 +1198,7 @@ fit.prophet <- function(m, df, ...) {
     X = as.matrix(seasonal.features),
     sigmas = array(prior.scales),
     tau = m$changepoint.prior.scale,
-    trend_indicator = as.numeric(m$growth == 'logistic'),
+    trend_indicator = switch(m$growth, 'linear'=0, 'logistic'=1, 'flat'=2),
     s_a = array(component.cols$additive_terms),
     s_m = array(component.cols$multiplicative_terms)
   )
@@ -1186,7 +1207,10 @@ fit.prophet <- function(m, df, ...) {
   if (m$growth == 'linear') {
     dat$cap <- rep(0, nrow(history))  # Unused inside Stan
     kinit <- linear_growth_init(history)
-  } else {
+  } else if (m$growth == 'flat') {
+    dat$cap <- rep(0, nrow(history)) # Unused inside Stan
+    kinit <- flat_growth_init(history)
+  } else if (m$growth == 'logistic') {
     dat$cap <- history$cap_scaled  # Add capacities to the Stan data
     kinit <- logistic_growth_init(history)
   }
@@ -1206,7 +1230,8 @@ fit.prophet <- function(m, df, ...) {
     )
   }
 
-  if (min(history$y) == max(history$y)) {
+  if (min(history$y) == max(history$y) & 
+        (m$growth %in% c('linear', 'flat'))) {
     # Nothing to fit.
     m$params <- stan_init()
     m$params$sigma_obs <- 0.
@@ -1318,6 +1343,19 @@ predict.prophet <- function(object, df = NULL, ...) {
   return(df)
 }
 
+#' Evaluate the flat trend function.
+#'
+#' @param t Vector of times on which the function is evaluated.
+#' @param m Float initial offset.
+#'
+#' @return Vector y(t).
+#'
+#' @keywords internal
+flat_trend <- function(t, m) {
+  y <- rep(m, length(t))
+  return(y)
+}
+
 #' Evaluate the piecewise linear function.
 #'
 #' @param t Vector of times on which the function is evaluated.
@@ -1392,7 +1430,9 @@ predict_trend <- function(model, df) {
   t <- df$t
   if (model$growth == 'linear') {
     trend <- piecewise_linear(t, deltas, k, param.m, model$changepoints.t)
-  } else {
+  } else if (model$growth == 'flat') {
+     trend <- flat_trend(t, param.m)
+  } else if (model$growth == 'logistic') {
     cap <- df$cap_scaled
     trend <- piecewise_logistic(
       t, cap, deltas, k, param.m, model$changepoints.t)
@@ -1592,7 +1632,9 @@ sample_predictive_trend <- function(model, df, iteration) {
   # Get the corresponding trend
   if (model$growth == 'linear') {
     trend <- piecewise_linear(t, deltas, k, param.m, changepoint.ts)
-  } else {
+  } else if (model$growth == 'flat') {
+    trend <- flat_trend(t, param.m)
+  } else if (model$growth == 'logistic') {
     cap <- df$cap_scaled
     trend <- piecewise_logistic(t, cap, deltas, k, param.m, changepoint.ts)
   }
diff --git a/R/inst/stan/prophet.stan b/R/inst/stan/prophet.stan
index 79db33c..7586125 100644
--- a/R/inst/stan/prophet.stan
+++ b/R/inst/stan/prophet.stan
@@ -27,7 +27,6 @@ functions {
   }
 
   // Logistic trend functions
-
   vector logistic_gamma(real k, real m, vector delta, vector t_change, int S) {
     vector[S] gamma;  // adjusted offsets, for piecewise continuity
     vector[S + 1] k_s;  // actual rate in each segment
@@ -62,7 +61,6 @@ functions {
   }
 
   // Linear trend function
-
   vector linear_trend(
     real k,
     real m,
@@ -73,6 +71,14 @@ functions {
   ) {
     return (k + A * delta) .* t + (m + A * (-t_change .* delta));
   }
+
+  // Flat trend function
+  vector flat_trend(
+    real m,
+    int T
+  ) {
+    return rep_vector(m, T);
+  }
 }
 
 data {
@@ -86,7 +92,7 @@ data {
   matrix[T,K] X;        // Regressors
   vector[K] sigmas;     // Scale on seasonality prior
   real<lower=0> tau;    // Scale on changepoints prior
-  int trend_indicator;  // 0 for linear, 1 for logistic
+  int trend_indicator;  // 0 for linear, 1 for logistic, 2 for flat
   vector[K] s_a;        // Indicator of additive features
   vector[K] s_m;        // Indicator of multiplicative features
 }
@@ -104,6 +110,17 @@ parameters {
   vector[K] beta;           // Regressor coefficients
 }
 
+transformed parameters {
+  vector[T] trend;
+  if (trend_indicator == 0) {
+    trend = linear_trend(k, m, delta, t, A, t_change);
+  } else if (trend_indicator == 1) {
+    trend = logistic_trend(k, m, delta, t, cap, A, t_change, S);
+  } else if (trend_indicator == 2) {
+    trend = flat_trend(m, T);
+  }
+}
+
 model {
   //priors
   k ~ normal(0, 5);
@@ -113,19 +130,10 @@ model {
   beta ~ normal(0, sigmas);
 
   // Likelihood
-  if (trend_indicator == 0) {
-    y ~ normal(
-      linear_trend(k, m, delta, t, A, t_change)
+  y ~ normal(
+    trend
       .* (1 + X * (beta .* s_m))
       + X * (beta .* s_a),
       sigma_obs
     );
-  } else if (trend_indicator == 1) {
-    y ~ normal(
-      logistic_trend(k, m, delta, t, cap, A, t_change, S)
-      .* (1 + X * (beta .* s_m))
-      + X * (beta .* s_a),
-      sigma_obs
-    );
-  }
 }
diff --git a/R/man/flat_growth_init.Rd b/R/man/flat_growth_init.Rd
new file mode 100644
index 0000000..a1a0002
--- /dev/null
+++ b/R/man/flat_growth_init.Rd
@@ -0,0 +1,22 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/prophet.R
+\name{flat_growth_init}
+\alias{flat_growth_init}
+\title{Initialize flat growth.}
+\usage{
+flat_growth_init(df)
+}
+\arguments{
+\item{df}{Data frame with columns ds (date), y_scaled (scaled time series),
+and t (scaled time).}
+}
+\value{
+A vector (k, m) with the rate (k) and offset (m) of the flat
+ growth function.
+}
+\description{
+Provides a strong initialization for flat growth by setting the
+growth to 0 and calculates the offset parameter that pass the 
+function through the mean of the the y_scaled values.
+}
+\keyword{internal}
diff --git a/R/man/flat_trend.Rd b/R/man/flat_trend.Rd
new file mode 100644
index 0000000..3ab1f5f
--- /dev/null
+++ b/R/man/flat_trend.Rd
@@ -0,0 +1,20 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/prophet.R
+\name{flat_trend}
+\alias{flat_trend}
+\title{Evaluate the flat trend function.}
+\usage{
+flat_trend(t, m)
+}
+\arguments{
+\item{t}{Vector of times on which the function is evaluated.}
+
+\item{m}{Float initial offset.}
+}
+\value{
+Vector y(t).
+}
+\description{
+Evaluate the flat trend function.
+}
+\keyword{internal}
diff --git a/R/man/linear_growth_init.Rd b/R/man/linear_growth_init.Rd
index 83815ad..d94b6f9 100644
--- a/R/man/linear_growth_init.Rd
+++ b/R/man/linear_growth_init.Rd
@@ -2,7 +2,7 @@
 % Please edit documentation in R/prophet.R
 \name{linear_growth_init}
 \alias{linear_growth_init}
-\title{Initialize linear growth.}
+\title{Initialize constant growth.}
 \usage{
 linear_growth_init(df)
 }
diff --git a/R/man/performance_metrics.Rd b/R/man/performance_metrics.Rd
index ef333bf..255da05 100644
--- a/R/man/performance_metrics.Rd
+++ b/R/man/performance_metrics.Rd
@@ -10,7 +10,7 @@ performance_metrics(df, metrics = NULL, rolling_window = 0.1)
 \item{df}{The dataframe returned by cross_validation.}
 
 \item{metrics}{An array of performance metrics to compute. If not provided,
-will use c('mse', 'rmse', 'mae', 'mape', 'mdape', 'coverage').}
+will use c('mse', 'rmse', 'mae', 'mape', 'mdape', 'smape', 'coverage').}
 
 \item{rolling_window}{Proportion of data to use in each rolling window for
 computing the metrics. Should be in [0, 1] to average.}
@@ -26,6 +26,7 @@ By default the following metrics are included:
 'mae': mean absolute error,
 'mape': mean percent error,
 'mdape': median percent error,
+'smape': symmetric mean absolute percentage error,
 'coverage': coverage of the upper and lower intervals
 }
 \details{
diff --git a/R/man/prophet.Rd b/R/man/prophet.Rd
index 618b351..10d4407 100644
--- a/R/man/prophet.Rd
+++ b/R/man/prophet.Rd
@@ -32,8 +32,8 @@ also have a column cap that specifies the capacity at each ds. If not
 provided, then the model object will be instantiated but not fit; use
 fit.prophet(m, df) to fit the model.}
 
-\item{growth}{String 'linear' or 'logistic' to specify a linear or logistic
-trend.}
+\item{growth}{String 'linear', 'logistic', or 'flat' to specify a linear, logistic
+or flat trend.}
 
 \item{changepoints}{Vector of dates at which to include potential
 changepoints. If not specified, potential changepoints are selected
diff --git a/R/man/smape.Rd b/R/man/smape.Rd
new file mode 100644
index 0000000..ce2e85f
--- /dev/null
+++ b/R/man/smape.Rd
@@ -0,0 +1,22 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/diagnostics.R
+\name{smape}
+\alias{smape}
+\title{Symmetric mean absolute percentage error
+based on Chen and Yang (2004) formula}
+\usage{
+smape(df, w)
+}
+\arguments{
+\item{df}{Cross-validation results dataframe.}
+
+\item{w}{Aggregation window size.}
+}
+\value{
+Array of symmetric mean absolute percent errors.
+}
+\description{
+Symmetric mean absolute percentage error
+based on Chen and Yang (2004) formula
+}
+\keyword{internal}
diff --git a/R/tests/testthat/test_diagnostics.R b/R/tests/testthat/test_diagnostics.R
index 0bf919d..465a89a 100644
--- a/R/tests/testthat/test_diagnostics.R
+++ b/R/tests/testthat/test_diagnostics.R
@@ -56,6 +56,18 @@ test_that("cross_validation_logistic", {
   expect_equal(sum((df.merged$y.x - df.merged$y.y) ** 2), 0)
 })
 
+test_that("cross_validation_flat", {
+  skip_if_not(Sys.getenv('R_ARCH') != '/i386')
+  df <- DATA
+  m <- prophet(df, growth = 'flat')
+  df.cv <- cross_validation(
+    m, horizon = 1, units = "days", period = 1, initial = 140)
+  expect_equal(length(unique(df.cv$cutoff)), 2)
+  expect_true(all(df.cv$cutoff < df.cv$ds))
+  df.merged <- dplyr::left_join(df.cv, m$history, by="ds")
+  expect_equal(sum((df.merged$y.x - df.merged$y.y) ** 2), 0)
+})
+
 test_that("cross_validation_extra_regressors", {
   skip_if_not(Sys.getenv('R_ARCH') != '/i386')
   df <- DATA
@@ -222,7 +234,7 @@ test_that("copy", {
   df$cap <- 200.
   df$binary_feature <- c(rep(0, 255), rep(1, 255))
   inputs <- list(
-    growth = c('linear', 'logistic'),
+    growth = c('linear', 'logistic', 'flat'),
     yearly.seasonality = c(TRUE, FALSE),
     weekly.seasonality = c(TRUE, FALSE),
     daily.seasonality = c(TRUE, FALSE),
diff --git a/R/tests/testthat/test_prophet.R b/R/tests/testthat/test_prophet.R
index 642b3c5..88469dc 100644
--- a/R/tests/testthat/test_prophet.R
+++ b/R/tests/testthat/test_prophet.R
@@ -237,9 +237,13 @@ test_that("growth_init", {
   expect_equal(params[2], 0.5307511, tolerance = 1e-6)
 
   params <- prophet:::logistic_growth_init(history)
-  
   expect_equal(params[1], 1.507925, tolerance = 1e-6)
   expect_equal(params[2], -0.08167497, tolerance = 1e-6)
+
+  params <- prophet:::flat_growth_init(history)
+  expect_equal(params[1], 0, tolerance = 1e-6)
+  expect_equal(params[2], 0.49335657, tolerance = 1e-6)
+
 })
 
 test_that("piecewise_linear", {
@@ -279,6 +283,19 @@ test_that("piecewise_logistic", {
   expect_equal(y, y.true, tolerance = 1e-6)
 })
 
+test_that("flat_trend", {
+  t <- seq(0, 10)
+  m <- 0.5
+  y = prophet:::flat_trend(t, m)
+  y.true <- rep(0.5, length(t))
+  expect_equal(y, y.true, tolerance = 1e-6)
+
+  t <- t[8:length(t)]
+  y = prophet:::flat_trend(t, m)
+  y.true <- y.true[8:length(y.true)]
+  expect_equal(y, y.true, tolerance = 1e-6)
+})
+
 test_that("holidays", {
   holidays <- data.frame(ds = c('2016-12-25'),
                         holiday = c('xmas'),
diff --git a/notebooks/additional_topics.ipynb b/notebooks/additional_topics.ipynb
index f3a9aa2..392a3cf 100644
--- a/notebooks/additional_topics.ipynb
+++ b/notebooks/additional_topics.ipynb
@@ -132,6 +132,16 @@
     "For time series that exhibit strong seasonality patterns rather than trend changes, it may be useful to force the trend growth rate to be flat. This can be achieved simply by passing `growth=flat` when creating the model:"
    ]
   },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "%%R\n",
+    "m <- prophet(df, growth='flat')"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": 5,
@@ -145,7 +155,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "This is currently implemented only in the Python version of Prophet. Note that if this is used on a time series that doesn't have a constant trend, any trend will be fit with the noise term and so there will be high predictive uncertainty in the forecast.\n",
+    "Note that if this is used on a time series that doesn't have a constant trend, any trend will be fit with the noise term and so there will be high predictive uncertainty in the forecast.\n",
     "\n",
     "To use a trend besides these three built-in trend functions (piecewise linear, piecewise logistic growth, and flat), you can download the source code from github, modify the trend function as desired in a local branch, and then install that local version. This PR provides a good illustration of what must be done to implement a custom trend: https://github.com/facebook/prophet/pull/1466/files."
    ]
@@ -237,4 +247,4 @@
  },
  "nbformat": 4,
  "nbformat_minor": 2
-}
+}
\ No newline at end of file