Most people are surprised to learn that loofah "sponges" grow in the garden, not undersea. They're actually squashes, sometimes called sponge gourds, and the blossoms and fruits are produced all summer. The mature loofah fruits actually look and grow a lot like cucumbers, and they're quite tasty while they're still small. Bug-free and easy to grow, these heat-loving vines are large and heavy so trellising is always necessary. Loofahs require about 140 to180 frost-free days to mature, so start them inside early if you're not located in Zone 7 or warmer.
Prepare your loofah gardening spot ahead of time. Plan to set the seedlings outside as soon as all danger of frost has passed for your area. Choose a rich, well-draining location in full sun. If the soil is poor, just amend it a couple of inches deep with some organic compost or peat moss. Install a sturdy trellis, or position your site in front of a fence. Otherwise, the large heavy vines will take over your entire gardening area. Loofahs will encircle and climb a wooden pole, or even a wall with a rough surface. They aren't terribly fussy about what they climb.
Fill the cells of a plastic seed starting six-pack to one quarter to one half an inch from the tops with seed starting mix. Set the six pack into a shallow container of warm water until the surface of the soil feels moist. Take it out of the water and let it drain for about an hour.
Plant two to three loofah seeds in each of the cells, about half an inch deep. Close the tray up in a clear plastic bag, and poke a few air holes in it with a toothpick. Set it in a warm spot with bright indirect light, such as on top of your refrigerator or over a hot water heater.
Check the seed tray each day to make sure that the soil never dries out. It should always be evenly moist, but never wet. If condensation forms on the inside of the plastic bag, open it up for a few hours to eliminate excess moisture. Make sure you're not over-watering. Your loofah seeds should germinate in two to four weeks.
Remove the plastic bag as soon as seedlings begin to poke through the soil. Move them to a very warm windowsill where they'll begin receiving direct sun. When two to four true leaves appear, choose the healthiest seedling in each cell, and cut the others off at the soil line.
Transplant the seedlings 8 to 12 inches apart in your prepared gardening site after all danger of frost has passed. Apply a 1-inch deep layer of lightweight mulch such as grass clippings, which is much better for loofahs than heavier materials like bark. Keep the mulch about 3 inches away from the stems to discourage rot.
Water just enough daily to keep the seedlings moist for the first week. Thereafter, watering once weekly should be sufficient as the seedlings become well established. Keep them uniformly moist, but don't let them stand in water, which will subject them to rot. Begin training them to climb their trellis or fence.
Apply an all-purpose liquid fertilizer when plants begin to produce yellow blooms around midsummer. Pinch off the first flush of flowers to appear, and snip off the first two to four lateral shoots from each plant. This will encourage fullness, as well as a higher production of better quality fruits. Monitor fruit production carefully, and cut off and discard any that are blemished or damaged.
Pick loofah gourds when they've fully matured, typically beginning in late fall. The brown or dark yellow fruits will be surprisingly lightweight for their size, and their skins will feel dry. They should remain on the vine as long as possible for best results. However, should a frost strike, pick any remaining loofahs right away, or they'll quickly rot.