Unheated Poly Tunnel, Wet Feet: Overwintering Container Stock With Gel

In the third week of January, a poly tunnel at 4°C and 90 percent humidity is the last place anyone worries about drought. Yet the nursery lost 30 percent of a bench of hebes that winter, and the post-mortem pointed somewhere unexpected: the potting media had been blended with 3 kg of gel per cubic metre back in spring, and by November every pot was holding water like a sponge that never got wrung out.

Gel earns its keep by stretching the interval between irrigations, and in a summer tunnel that is exactly what a grower wants. Through a cold winter the same property works in reverse. Evaporation is near zero, the crop drinks almost nothing, and the water the gel released in autumn stays in the pot. Cold, saturated media is the standard recipe for root rot, and the grower who blended in the gel in April is rarely the one who connects it to the losses in January.

What cold, wet gel media actually does

Three problems stack up. First, disease: media at field capacity for weeks is where the root pathogens win, and the poly tunnel’s still air makes botrytis a second front. Second, temperature: water conducts heat out of the root ball far faster than air-filled pore space does, so wet pots sit colder at root level than dry ones, and cold plus wet is precisely the combination most stock refuses to tolerate. Third, weight: a gel-loaded media holds several extra litres per cubic metre, which matters on benches, and matters even more on rolling benches rated for a certain load per metre.

The fix is seasonal, not chemical. From November, irrigation on gelled media should be cut hard, often to once every ten days or less, guided by pot weight rather than the calendar. The gel keeps the media from reaching the wilting point even at that sparse schedule, which is its real winter value: it lets a grower water far less without any pot actually drying out.

Setting rates for stock that overwinters

• Summer production media: 1.5-3 kg per cubic metre is the usual working range.
• Stock destined for an unheated tunnel winter: 1-1.5 kg per cubic metre, or blend the overwinter blocks separately.
• From November: irrigate on pot weight, roughly when the pot has lost a third of its saturated weight.
• Ventilate on dry mornings even in frost; moving air is the cheapest disease control there is.

The counter-intuitive lesson from the hebe bench is that the autumn recommendation to ‘keep everything evenly moist’ is written for open ground, not for a closed tunnel with a wetted polymer in every pot. Evenly moist in a cold tunnel means evenly at risk. A moisture meter stuck into a gel-blended pot reads high all winter and will mislead anyone using it to schedule; lifting the pot tells the truth.

The spring restart

Winter discipline ends the day the tunnel starts moving again, and the transition deserves its own check. In March, lift and weigh a pot from each overwinter block before the first full irrigation; a media that has held its structure and smells clean will restart on the normal schedule, while any block that stays waterlogged a day after a test watering needs its drying cycle repeated before the season’s feeding starts.

It is also the moment to reconcile the blend against the bench plan. Overwinter stock that performed well on a 1 kg per cubic metre media tells you something for the next spring’s mixing, and stock that struggled tells you more. The growers who treat each March as an audit of the previous November’s decisions stop repeating the hebe-bench failure; the ones who simply resume last year’s schedule are the ones who meet it again next January.

Before the first frost, walk the overwinter blocks and check each media blend’s gel rate against the bench plan, then write the winter irrigation interval on the tunnel door. The media and dose guidance behind these numbers is set out in the water retaining gel application guide, and a one-page winter schedule per tunnel is worth more than any amount of January diagnosis.