diff --git a/libexec/rc/rc.d/growfs b/libexec/rc/rc.d/growfs
--- a/libexec/rc/rc.d/growfs
+++ b/libexec/rc/rc.d/growfs
@@ -1,5 +1,6 @@
 #!/bin/sh
 #
+# Copyright 2022 Michael J. Karels
 # Copyright 2014 John-Mark Gurney
 # All rights reserved.
 #
@@ -63,6 +64,7 @@
 
 growfs_start ()
 {
+	verbose=0
 	echo "Growing root partition to fill device"
 	FSTYPE=$(mount -p | awk '{ if ( $2 == "/") { print $3 }}')
 	FSDEV=$(mount -p | awk '{ if ( $2 == "/") { print $1 }}')
@@ -100,19 +102,111 @@
 		diskdev=${rootdev}
 	fi
 
+	# Check kenv for growfs_swap_size; if not present,
+	# check $growfs_swap_size from /etc/rc.conf.
+	# A value of 0 suppresses swap addition,
+	# "" (or unset) specifies the default.
+	addswap=1
+	swapsize="$(kenv -q growfs_swap_size 2>/dev/null)"
+	case "$swapsize" in
+	"0")	addswap=0
+		;;
+	"")	case "$growfs_swap_size" in
+		"0")	addswap=0
+			;;
+		"")	;;
+		*)	swapsize="$growfs_swap_size"
+			;;
+		esac
+		;;
+	*)	;;
+	esac
+
+	[ $verbose -eq 1 ] && {
+		echo "diskdev is $diskdev"
+		echo "search is $search"
+		echo "swapsize is $swapsize"
+	}
+
 	sysctl -b kern.geom.conftxt | awk '
 {
+	verbose = 0
 	lvl=$1
 	device[lvl] = $3
 	type[lvl] = $2
 	idx[lvl] = $7
+	offset[lvl] = $9
 	parttype[lvl] = $13
+	size[lvl] = $4
+	if (verbose) print lvl, type[lvl], $3
+	if (type[lvl] == "DISK") {
+		disksize = size[lvl]
+		if (verbose)
+			print "disksize ", disksize
+		if (addswap == 1 && (size[lvl] > 15000000000 || swapsize > 0))
+			doing_swap = 1
+		else
+			doing_swap = 0
+	} else if (type[lvl] == "PART" && $11 == "freebsd-swap" && \
+	    int(swapsize) == 0) {
+		# This finds swap only if it precedes root, e.g. preceding disk.
+		addswap = 0
+		doing_swap = 0
+		print "swap device exists, not adding swap"
+	}
 	if (dev == $3) {
 		for (i = 1; i <= lvl; i++) {
 			# resize
 			if (type[i] == "PART") {
 				pdev = device[i - 1]
-				cmd[i] = "gpart resize -i " idx[i] " " pdev
+				if (verbose)
+					print i, pdev, addswap, disksize, \
+					    doing_swap
+				swapcmd = ""
+				# Allow swap if current root is < 40% of disk.
+				if (parttype[i] != "MBR" && doing_swap == 1 && \
+				    (size[i] / disksize < 0.4 || \
+				    swapsize > 0)) {
+					print "Adding swap partition"
+					sector = $5
+					if (int(swapsize) == 0) {
+						swapsize = \
+						    int(disksize / sector / 10)
+						swaplim = 2 * physmem / sector
+						if (swapsize > swaplim)
+							swapsize = swaplim
+					}
+					if (verbose)
+						print "swapsize ", swapsize
+					align = 4 * 1024 * 1024 / sector
+
+					# Estimate offset for swap; let
+					# gpart compute actual start and size.
+					# Assume expansion all goes into this
+					# partition for MBR case.
+					if (parttype[i - 1] == "MBR") {
+					    if (verbose)
+						print "sz ", size[i - 1], \
+						    " off ", offset[i - 1]
+					    expand = size[0] - \
+						(size[i - 1] + offset[i - 1])
+					} else {
+					    if (verbose)
+						print "sz ", size[i], \
+						    " off ", offset[i]
+					    expand = size[0] - \
+						(size[i] + offset[i])
+					}
+					if (verbose)
+					    print "expand ", expand, \
+						" sz ", size[i]
+					swapbase = (expand + size[i]) / sector
+					swapbase -= swapsize + align
+					swapcmd = "gpart add -t freebsd-swap -a " align " -b " swapbase " " pdev "; kenv growfs_swap_added=1 >/dev/null; "
+					if (verbose)
+						swapcmd = "set -x; " swapcmd
+				}
+				cmd[i] = swapcmd "gpart resize -i " idx[i] " " pdev
 				if (parttype[i] == "GPT")
 					cmd[i] = "gpart recover " pdev " ; " cmd[i]
 			} else if (type[i] == "LABEL") {
@@ -128,7 +222,8 @@
 		}
 		exit 0
 	}
-}' dev="$search"
+}' dev="$search" physmem="$(sysctl -n hw.physmem)" addswap="$addswap" \
+   swapsize="$swapsize"
 	gpart commit "$diskdev" 2> /dev/null
 	case "$FSTYPE" in
 	ufs)