Should I grow in the greenhouse border or in pots?

This decision drives almost everything else about how a greenhouse is run: how often you water, how you feed, whether you take a holiday in July, and whether the crop declines after year four. There is no universally right answer, but there is a right answer for how much time you can give it.

Step by step

1. Assess how often you can water.Border soil can go two or three days in warm weather. Large pots need daily attention. Growbags need daily and sometimes twice daily. If nobody is at home midweek, that decides it before anything else does.
2. Consider what you grow and how often.If you grow tomatoes in the same border every year, soil sickness is a question of when, not if. If you rotate — tomatoes one year, cucumbers or a green manure the next — the border lasts much longer.
3. Use the border while it is good.A new or refreshed border will out-crop pots for several years. Improve it with garden compost each autumn and feed as the crop demands. Do not abandon it prematurely.
4. Watch for the decline.Smaller plants, yellowing lower leaves, wilting in sun despite moist soil, and a crop noticeably worse than last year on the same variety and the same care. That is soil sickness, and it means changing the system or changing the soil.
5. Switch to ring culture rather than to pots.When the border tires, standing rings on a gravel bed over the old soil keeps most of the water buffering while isolating the plant's feeding roots from the problem. It is the traditional answer and it works.
6. Use pots for the plants that need isolation.Anything you have had disease on, anything you are trialling, and anything you want to move outside later. Pots also let you use a different compost for a different crop in the same house.
7. Feed and water according to the system, not to habit.A border needs less frequent, more thorough watering and less feeding. A growbag needs feeding at almost every watering once fruit sets. Applying border practice to pots starves them, and pot practice to a border overfeeds it.

Tips

• A hybrid layout — border down the sunny side, staging and pots down the other — is what most experienced greenhouse growers end up with.
• If you build a raised bed inside the greenhouse, make it deep enough to matter. 30cm minimum, 40cm better, and line the timber the same way you would an outdoor planter.
• Green manure sown in the border in autumn and dug in during winter genuinely helps keep border soil in condition between crops.

Common mistakes

• Growing tomatoes in the same border soil every year indefinitely — It is the direct cause of greenhouse soil sickness. Rotate, use ring culture, or replace the soil every few years.
• Choosing pots and then watering like a border — A large pot in a greenhouse in July can use several litres a day. Watering every third day produces split fruit, blossom end rot and a stalled plant.
• Filling a greenhouse border with bagged multipurpose compost — It collapses within a season and holds far too much water at depth. Border soil should be topsoil improved with compost, not compost alone.

Alternatives

• Greenhouse border soil: The largest root volume available, good moisture buffering, and the least daily watering. Best crops in the first few years. Accumulates root disease, eelworm and nutrient imbalance when the same crop is grown repeatedly, which is what the soil sickness problem is.
• Large pots, 30 litres and up: Complete disease isolation, fresh compost every year, and total control of feeding. Needs watering at least daily in summer and twice on hot days, and the plants blow over unless supported.
• Growbags: Cheap and convenient, but only about 150mm of compost, so they are the most demanding of all for watering. Best used in a tray, or with ring culture on top.
• Ring culture: A bottomless pot standing on a bed of gravel or on a growbag. Feeding roots in the ring, water-seeking roots below. Effectively doubles the root run, isolates the plant from border soil, and is far more forgiving than pots alone.
• Raised bed inside the greenhouse: A timber or block bed filled with imported compost and topsoil, standing on the greenhouse floor. Gives border-like performance with soil you control and can replace, without excavating.

Questions people ask