A half-inch of crumbly mature compost spread over a lawn or around a shrub in a heavy container vanishes within weeks as rain, watering and earthworms draw it down. That is topdressing: a fine, soil-like layer that rejuvenates tired ground without the shock of repotting, provided you use rich material such as mature compost rather than nutrient-poor seedling mix.
Top dressing is for lawns, plants in large containers or pots, along edges, in raised garden beds and even under trees that like rich soil.
You can also use compost, which has fertilizing properties and will cover the needs of your plants for at least a year.
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Topdressing is a technique or practice where a layer of fresh, extra soil is added on top of existing soil. The goal is to rejuvenate soil and replenish its nutrients.
“Topdressing” is also the noun that designates the material or amendment itself (usually soil mix or sand).
Topdressing is normally spread on the surface rather than dug in, letting rain, watering and earthworms carry it down, though gardeners often rake or brush it lightly into lawns and around plants.
Do not confuse it with mulching, which lays a coarse protective cover on the surface, while topdressing adds a fine, soil-like layer that settles in and disappears.
Side-dressing is a different operation again: fertiliser placed in a band alongside a row or plant, not a layer spread over the whole surface.
Topdressing a plant or lawn is a good way to care for it. It helps reduce the need for frequent fertilizing.
Furthermore, there are positive side effects:
Unlike repotting, topdressing is softer on the plant. It won’t suffer any form of transplant shock.
How much material you spread, and how often you come back to it, depends entirely on what you are topdressing. The three commonest cases in this article line up as follows.
| Situation | Layer or amount | How often | Best season |
|---|---|---|---|
| Lawn | Very thin layer over the whole surface, ¼ to ½ inch (0.5 to 1 cm) | Up to four times a year | Early to mid-spring and/or early to mid-autumn |
| Houseplant in a large pot | Dig out the old soil until the root ball shows, refill to the crown | Every two years | Start of the growing season, usually early spring |
| Raised bed or garden box | About ½ inch (1.5 cm), roughly 6 pounds (3 kg) per square yard or metre | Every three or four years | Early spring, as growth begins |
Whichever case applies, keep the layer thin and the crown of the plant clear.
Circumstances when topdressing is expected
Topdressing usually comes into play when repotting isn’t possible.
The following situations are typical topdressing applications:
For lawns and growing beds in shallow soil, the question is clear cut.
For houseplants, container gardens, raised beds and the like, it’s more a question of convenience.
Ideally, topdress with these weather conditions:
The goals is to spread a very thin layer of soil mix across the entire surface of the lawn.
Although it’s possible to simply throw the soil around with a wide movement of the arm (as if you were broadcast sowing seeds), there are other techniques that produce better results with specific equipment.
Some indoor plants require rather large pots, such as indoor banana or citrus trees. Since repotting them is almost impossible due to size, the next best option is to topdress them.
Usually a large plant will benefit from being topdressed every two years. You can also support growth with natural fertilizer such as fermented tea.
As long as there’s still space under the rim of the growing bed, you can topdress very easily simply by adding a thin layer of topdressing at the top.
Topdressing in such a manner is best repeated every three or four years. You can complement topdressing with a layer of plant mulch. Plant mulch is ideal for raised beds because it breaks down and continuously releases nutrients.
When the garden box is full, which may happen after 4 to 6 years, you’ll have to go for soil replacement. This is similar to what is described just above for topdressing houseplants.
There are a few points of caution regarding this technique.
Delicate hands – When rummaging around in pots, tools will often wound roots. This raises risks of infection and root rot. Use your hands to work the soil when working on houseplants.
Poor topdressing – the goal is to replenish nutrients. Best is to use rich materials like compost. Sometimes potting mix is too poor. For example, you shouldn’t use seedling potting mix because there are very few nutrients in it.
Prefer compost that is mature and cool to the touch, because immature, carbon-rich compost can temporarily tie up soil nitrogen, especially when dug in rather than spread as a thin surface topdressing.
Deep soil piled against a plant’s crown or trunk traps moisture and invites rot, so keep the crown itself clear, limiting any topdressing to a thin layer feathered away from the stem.
Topdressing will not rescue a pot that has become solid root: if roots spiral out of the drainage holes, repot or root-prune instead.
On a lawn, brush the topdressing in with the back of a rake so the grass tips stay visible and are not smothered.
For most purposes, mature compost is the best topdressing.
Other possibilities include:
Sand – this is almost exclusively used as a lawn topdress, and then again mostly on golf turf. You may include it in your topdressing mix (up to a third) if you need to increase drainage, but don’t repeat it too often. Prefer coarse sand to fine sand. Never use sea sand which is salty. Only use river sand or quarry sand.
Manure – manure provides very rich nutrients. Depending on what the animals ate, though, you might notice weeds growing out of it (especially grasses).
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Topdressing is an excellent organic option to maintain your plants. You don’t need to add any chemical fertilizers because nutrients are replenished with fresh soil.
