Floral foam is a lightweight, porous block that holds flower stems in place and supplies them with water. Florists also call it florist foam, flower foam, wet foam, green foam, flower sponge or oasis. It lets you build an arrangement at any angle without a vase holding the stems upright.

It comes in two families: wet foam for fresh flowers, and dry foam for artificial and dried stems.

Floral foam is a phenolic foam, produced by expanding a phenol-formaldehyde resin into an open-cell structure. Those open cells are what let it draw up and hold many times its own weight in water while staying rigid enough to grip a stem.

Dry foam for artificial flowers is usually a denser polystyrene-based foam, which is why it does not absorb water.

Its proper name is floral foam. You will also hear florist foam, flower foam, green foam, wet foam, floral sponge, flower sponge, and oasis — the last of which is a brand name used generically, much like hoover or biro. In Spanish it is esponja verde para flores.

All of these refer to the same product.

Cut the block to fit your container, soak it if you are using fresh flowers, then seat it firmly so it cannot rock. Cut each stem at an angle and push it in once, to the depth you want. Every stem leaves a permanent hole, so place deliberately.

• Work from the outside edges inward, then fill the top.
• Leave a small water reservoir below the foam so you can top it up.
• Do not re-insert a stem into the same hole — it will not draw water.

Float the block on the surface of a deep container of water and let it sink under its own weight. A standard brick usually settles in under a minute, and it is ready the moment the top surface is level with the water and the colour has darkened all over.

Never push it under. Forcing it traps air and leaves dry pockets in the centre, and any stem landing in one gets no water at all.

A fully soaked block typically stays usefully damp for several days indoors, though the real answer depends on how thirsty your flowers are, how warm the room is, and how much foam is exposed to the air.

Top up the reservoir daily rather than waiting for the foam to dry. Once floral foam dries out completely it will not reabsorb water properly, and the arrangement is effectively finished.

With the foam kept topped up, most arrangements hold well for several days to about a week — comparable to the same flowers in a vase, and often longer for an installation that could not stand in water at all.

Longevity comes down to the flowers themselves, a clean angled cut on every stem, and never letting the foam dry out.

Cut it before you soak it, using a long straight blade or a florist knife drawn in one smooth stroke rather than a sawing motion. Measure against the container first — a block that fits snugly needs no tape.

Cutting dry foam raises fine dust, so work in a ventilated space and wear a dust mask. Wetting the surface first keeps the dust down considerably.

No. Dry foam does not absorb or hold water, so fresh stems placed in it have nothing to drink and will wilt within hours. It is made for artificial, silk and dried arrangements, where its firmer grip is an advantage.

For fresh flowers you need wet floral foam, which is designed to soak and hold water.

Use dry foam and leave it dry. Cut it to fit the container, wedge it firmly, then push the wire stems straight in — dry foam grips harder than wet foam, so heavier silk stems stay exactly where you put them.

Cover the surface with moss, stones or foliage to hide it, and the arrangement will hold its shape indefinitely.

Not for fresh flowers. Every stem leaves a hole, and a used block is riddled with channels that break the capillary structure, so water no longer reaches new stems reliably. Old blocks also harbour bacteria that shorten the life of the next arrangement.

Dry foam used for artificial stems is a different case and can often be reused several times, since nothing has to travel through it.

Floral foam is a phenolic resin foam. It is not biodegradable and it does not break down in landfill, and we would rather tell you that plainly than claim otherwise.

In normal use it is not classified as acutely hazardous, but sensible handling matters: cut it in a ventilated space, wear a mask when cutting it dry, wash your hands afterwards, and keep it away from children and pets. Do not pour foam-laden water down the drain — bin the solids.

If sustainability is a priority for a particular job, chicken wire, kenzan pin holders and reusable mechanics are genuine alternatives. Foam earns its place where nothing else will hold stems at an angle and keep them drinking — arches, hanging installations and large event work.