Standard multipurpose compost is designed to suit a broad range of plants, which often means it holds more moisture than certain container plants would actually prefer, particularly those used to drier, more open growing conditions in the wild. Mixing in a proportion of perlite lightens the overall texture and creates more space for air and water to move through freely. This is especially useful in pots that don't drain quite as freely as they should.
Perlite is a lightweight, naturally occurring material that has been heated until it expands into small, porous white granules, which sit within the compost and prevent it from becoming too dense or compacted over time. Unlike sand, it doesn't add significant weight to the pot, which is a useful advantage for balcony containers where weight matters. It also tends to be far easier to handle than heavier alternatives when filling several pots at once.
How much perlite to use and when
For plants that particularly dislike wet roots, such as many herbs and drought-tolerant species, a generous proportion of perlite mixed thoroughly through the compost gives noticeably better results than compost used straight from the bag. For thirstier plants, a smaller proportion is usually enough to improve drainage without drying the compost out too quickly between waterings.
Mix the perlite in thoroughly before planting, rather than simply scattering it on the surface afterwards, so that the improved drainage is consistent throughout the whole depth of the pot rather than concentrated near the top. A quick mix in a bucket or on a spread-out sheet works well for smaller batches. Wearing gloves is worth considering, since the fine dust from the bag can be a little irritating.
Once mixed in, perlite doesn't break down the way organic matter does, so its benefits to drainage continue for as long as the compost itself remains in use. It's a small, inexpensive change that can make a noticeable difference to how well container plants cope with a wet spell.
