Topdressing, what is it?

Topdressing means spreading a fresh layer of rich soil mix directly atop the ground. This gives plants the organic matter they need to grow best.

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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What is topdressing?

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).

  • You topdress a plant with topdressing mix.

Advantages of topdressing

Topdressing a plant or lawn is a good way to care for it. It helps reduce the need for frequent fertilizing.

  • This operation helps soil become richer in terms of organic matter.
  • It also helps soil stay airy and light, fighting the natural tendency of soil to grow heavier over the years.
  • Topdressing enhances soil structure.
  • Soil that is well structured has superior water retention properties and is a more fertile environment for plants.
  • When you add soil mix or compost, the ground grows more porous. This maximizes local gaseous exchanges that promote proper root growth.

Furthermore, there are positive side effects:

  • no need to fertilize during the first year
  • water needs are reduced
  • containers and pots seem refreshed and don’t need ornamental mulch for a while.

Unlike repotting, topdressing is softer on the plant. It won’t suffer any form of transplant shock.

When to topdress

Topdressing is called for in this heavy marble pot.Circumstances when topdressing is expected

Topdressing usually comes into play when repotting isn’t possible.

The following situations are typical topdressing applications:

  • Large containers and pots
  • Tall or wide plants that are difficult to move, even if in manageable pots
  • Wide surfaces such as lawns
  • Raised gardens of any type: raised beds, keyhole gardens, waist-high planters…
  • City gardens or urban growing beds where plants are locked up in masonry
  • Tree beds with shallow soil due to either high bedrock or a high water table

Best season to topdress

For lawns and growing beds in shallow soil, the question is clear cut.

  • Topdress in spring and/or fall, early in the season and/or mid-season.
  • You can topdress up to four times a year.
  • Don’t topdress in summer or winter.

For houseplants, container gardens, raised beds and the like, it’s more a question of convenience.

  • Topdress when the plant shows symptoms of lacking nutrients.
  • As a regular course of action, topdressing every two years is a good rule.
  • The best season is at the beginning of the growing season. Usually this is the beginning of spring for most plants, but some are late starters and late spring is fine.

Best weather to topdress

Ideally, topdress with these weather conditions:

  • after at least a week of no rain (or no watering for houseplants). The topdressing mix is thus dry and easy to spread on ground that is also dry. It won’t cake up. Clumps crumble easily and sift under leaves to the surface.
  • just before a bout of rain. Avoid periods of strong torrential rains which will wash most of the topdressing away. Favor times of soft, light rain or day-long drizzling which is perfect!

How to topdress plants

Topdressing a lawn

The goals is to spread a very thin layer of soil mix across the entire surface of the lawn.

  • This layer should only be ¼th to ½ inch thick, or 0.5 to 1 cm.

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.

Topdress a houseplant

Hands working to topdress a houseplantSome 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.

  • Bring the pot over an easy-to-clean surface like tiles or spread a tarp under it.
  • Gently dig out soil out from the top with your hands. You can break the soil up a bit with a cultivator, but it’s best not to use tools because roots may be damaged. A dull spoon or wooden ladle helps, though.
  • Remove dirt until the root ball is exposed (topmost roots showing).
  • Fill in the space you made free with topdressing mix.
  • Press down, and keep filling until you reach the crown of the plant, where the soil level used to be.
  • Water and if more space is available, add a few final handfuls of topdressing mix.

Usually a large plant will benefit from being topdressed every two years. You can also support growth with natural fertilizer such as fermented tea.

How to topdress a large garden box or a raised bed

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.

  • Break the crust on the surface, if any, with a cultivator.
  • Compute the amount of topdressing you need. Per square yard or meter, around 6 pounds (3 kg) usually is enough. For raised gardens and garden boxes, it should add up to ½ inch (1.5 cm) of topdressing at the top.
  • If you’ve got shrubs and plants growing there, don’t add soil right near the crown. Stay about an inch away from the stem or trunk on all sides.
  • Water lightly, enough to moisten the topdressing but not flooding the bed. Too much water would displace the layer of topdressing and make it uneven.

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.

The crown is the part of a plant where stem meets roots. It is where many exchanges take place.

Topdressing precautions

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.

Which topdressing is best

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).

Smart tip about topdressing

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.

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Credits for images shared to Nature & Garden (all edits by Gaspard Lorthiois):
Raking topdress on lawn by Rosalyn & Gaspard Lorthiois, own work
Large heavy container needing topdressing by Tanuj Handa ★ under Pixabay license
Working hands by Gabriel Jimenez ★ under Unsplash license
Crown of a plant by Robert Owen-Wahl ★ under Pixabay license