Terrace gardens fail for three common reasons: heavy soil that does not drain, pots that dry out in an afternoon, and feeding that is either missing or too much. A good mix and a steady, modest feed solve most of it.
Check the roof first
Wet soil is heavy whatever you feed it with. ICAR's Indian Horticulture journal gives two ranges: an extensive roof garden, 10 to 20 cm of growing medium, loads a roof with 72 to 169 kg per square metre, and an intensive one, 20 to 100 cm deep, with 250 to 950 kg. Have the roof checked and waterproofed before you load it, and choose a light mix.
The mix
A light terrace mix, by volume
| Part | Share | What it does |
|---|---|---|
| Garden soil or potting soil | About two parts | Holds the roots and some nutrients |
| Cocopeat | About one part | Keeps the mix light and holds water |
| Vermicompost | One part or less (10 to 25%) | Food and microbes; extension services cap it at about a quarter |
ICAR notes that cocopeat is popular for roof gardens and that home compost or vermicompost is the ideal addition. Keep the share of vermicompost at a quarter or less: a mix that is too rich holds more salt than young roots like.
Feeding through the season
- In an established pot, scratch 100 to 200 g of vermicompost into the top few centimetres every 30 to 45 days, then water.
- Water deeply and less often, and check the soil with a finger first.
- Do not treat a compost tea or a vermiwash as a substitute. The University of California does not recommend them on edible crops, and the evidence for them is thinner than for vermicompost itself.
No Indian university has published a tested per-pot feeding calendar for terrace crops, so the schedule above follows the standard rates and should be adjusted by how the plants look.
What to grow first
ICAR's guidance is to raise brinjal, tomato and chilli seedlings in trays for 25 to 30 days and then move them to polybags or pots. Okra, cluster bean, bitter gourd, bottle gourd and radish are sown directly where they will grow.
Common questions
What is the best organic fertiliser for a terrace garden?
A light potting mix with vermicompost as part of it, topped up with a handful of vermicompost every month, is a sound base. Well-made compost and farmyard manure also work. No single product is best for every plant.
How much vermicompost per grow bag?
Mix 10 to 25 percent by volume into the filling, then feed 100 to 200 g every 30 to 45 days for a bag that holds about the volume of an 8-inch pot, and more in proportion for bigger bags.
Will vermicompost make my terrace smell?
Good vermicompost smells of earth. The Fertiliser (Control) Order asks for an absence of foul odour. A sharp ammonia or rotten smell means the material is immature, and it should not be used.
Is vermicompost heavy for a roof?
The weight comes from the wet growing medium as a whole, and vermicompost is only part of it. Plan by the depth and area of the mix, using ICAR's load figures, and have the roof checked.
How often should I feed terrace plants?
Every 30 to 45 days in the growing season is the standard rate for potted plants.
Terms in this guide
Sources
- ICAR, Indian Horticulture: rooftop gardening (1)
- ICAR, Indian Horticulture: rooftop gardening (2)
- ICAR Research Complex for NEH Region: vermicompost production and practices
- Oregon State University Extension: using compost
- University of Maryland Extension: worm composting
- University of California: compost teas and extracts