The dry start method, or DSM, is an aquascaping technique for establishing a thick, even carpet of moss or foreground plants before the tank ever holds water. You plant into damp hardscape, seal the tank to trap humidity, grow everything emersed for a few weeks, and only then flood it. It takes patience up front and repays it with a dense, algae-free carpet that would be much harder to grow underwater from the start.
Why it works
Many carpeting plants and mosses actually grow faster out of the water than in it, given saturated air, because carbon dioxide is freely available from the atmosphere rather than limited as it is when dissolved. Just as importantly, while the tank is drained there is no water column for algae to bloom in, so the young plants get a clear run to establish without competition. By the time you flood, the carpet is rooted, dense and able to hold its own.
Setting it up
Lay your substrate and hardscape and shape the scape as you want it. Plant moss as a thin scatter of fragments pressed firmly onto the wood, rock or soil, and tuck carpeting plants in small, well-spaced clumps. Mist everything thoroughly with dechlorinated or RO water until the surface glistens and a shallow film sits in the low points, but do not flood. Then seal the top with cling film or glass to hold near-total humidity, and light the tank on a normal photoperiod, around eight to ten hours.
The waiting weeks
For the next few weeks the work is light: keep the surface damp by misting if it dries, lift the cover briefly every day or two for fresh air to stave off mould, and watch for growth. You will see fine new shoots and spreading first, then thickening. Pull out any fuzzy mould promptly and air the tank more if it appears. Most scapes need around four to six weeks emersed, sometimes longer for a really dense carpet; rushing it is the usual mistake.
When and how to flood
Flood once the carpet has knitted together and is gripping the surface, not before. Add water slowly and gently, ideally pouring onto a plate or bag laid on the substrate so you do not blast the new growth loose. Expect a short adjustment as the plants switch from emersed to submerged growth, during which a little melt-back is normal, and run good flow and your normal maintenance from day one to keep early algae down.
The trade-offs
The DSM gives a denser, more even, more securely attached carpet with far less early algae than planting straight into water, and it is gentle on the wallet because nothing is lost to a tank that has not cycled. Against that, it is slow, it ties the tank up for weeks before any fish go in, and not every plant takes the transition to submerged life equally well. For moss specifically it works very well, and it pairs naturally with the bulk-growing ideas in how to propagate aquarium moss and the rig in the propagation box.