Help, my greenhouse plants don’t look great, what should I do?

As our greenhouses move into the peak months of production the actual plants can almost burn themselves out by growing, flowering and fruiting in abundance. They have one objective in life, it’s totally programmed into them and that’s to flower, fruit and set seed. Great news if you are growing fruiting plants such as tomatoes, cucumbers, aubergines, beans and others, as you want the plants to be as productive as possible. But as the plants turn from neatly trained greenhouse specimens into greenhouse Triffids any thoughts of low maintenance care must be cast out. Forget a holiday in July unless you’ve kitted your greenhouse out with the latest hi tech watering kit, auto vents or have a tame friend or neighbour that can step in and take over. Your plants need constant care.

First they need good ventilation and if you can’t provide this then you risk a variety of problems from overheating to fungal issues, in particular botrytis, which thrives in these conditions. Stems collapsing, grey furry leaves and fallen seedlings are all signs and symptoms.

Before it gets to that stage, check the windows can be opened and the openings protected against entry by unwelcome visitors such as cats, rats and anything else. A mesh screen is ideal. Top vents should be automated for ease of use and to operate in early morning or during periods of unexpected sunshine. But don’t forget the bottom vents, there needs to be a flow of air from top to bottom. This draws cool air in low down and releases hot air high up.

To keep your plants growing healthily they need plenty of root space. From seedling to mature plants, many greenhouse plants are potted up two, three or more times into sequentially bigger pots. It’s not a good idea to plant a tiny seedling into a large planter, you might think you can save yourself time, but often the result is failure with that plant, as the excess compost sours and the plant roots don’t grow into it. If your greenhouse plants are root bound in small pots they will not thrive either. Repot them into larger planters, growing bags or containers using quality potting compost. The better the compost the better your results.

You need to tailor your watering to suit their growth stage and the weather. In July if the weather is hot your plants will need lots of water, but if it cools off it’s a good idea to let the compost dry out a little in between watering. This avoids root damage and root rots. Self watering containers and planters are great when your plants are at their peak and needing lots of water, but used too soon in the season, they can encourage wet compost, scariad flies and other wet compost issues. Set up your plants on top self-watering trays but restrict their use, i.e. water the plants and not the trays until the weather hots up or you are away and need the self watering facility.

Ailing greenhouse plants are often a symptom of a poor feeding regime. You can’t get away without feeding your plants if you want good results. They will grow, but they simply won’t perform. If your plants are yellow or looking sickly first make sure that they are not too wet and that the roots are healthy. If they are still in small pots then the chances are they are starving, repot into a good potting compost and add additional feed into the compost such as a slow release or a controlled release granule. If you need an emergency boost then feed with a diluted liquid feed. Do not overfeed or you will scorch the plant roots. Many physiological disorders in greenhouse plants are a result of poor feeding or nutrient deficiencies.

Finally make a check that your plants are not under attack. Greenhouses are a protected environment for not just your plants, all sorts of pest sand diseases will thrive in the greenhouse too. Insect pests should be obvious. Flying pests such as whitefly can wreak havoc on greenhouse crops. Aphids suck sap and transmit all sorts of plant viruses. Then there’s red spider mite, mealy bug, caterpillars, vine weevils and of course the slugs and snails, not to mention plant munching rodents and worse. It’s not all bad news; you’d be very unlucky to suffer from all of these problems. Try and work out what the root of the problem is, get some advice from another greenhouse gardener if you are not sure and then make a note to do better next year. Most problems can be overcome and most gardeners have some sort of crop failure every year, no matter how experienced they are.

How can I cool my summer greenhouse?

The July greenhouse is a hot and humid place. Well, in a normal summer anyway. Having a greenhouse and keeping it cool is a bit of a dichotomy. The very nature of the greenhouse harnesses the solar energy to warm the greenhouse interior. That’s what they are designed for. In winter, spring and even in autumn this is what enables the greenhouse gardener to grow out of season, but in the heat of the summer the greenhouse can be a very hot place indeed.

First you need to consider your plants, many will thrive very happily in a hot and humid environment and any that are ailing in the heat or showing signs of scorched leaves could be moved outside if practical.

Then you need to look at ventilation, at this time of year the greenhouse door takes on a new role and where possible should be secured open for most of the day but especially mid morning to mid afternoon to allow good airflow.

You need as many greenhouse vents as possible. Ideally a greenhouse should have a third of its footprint area available as opening vents. It’s not easy to add more ventilation to an existing greenhouse, but in some greenhouses it may be possible to remove a pane of glass to increase airflow. Remember that the natural airflow draws cool air into the greenhouse low down and releases the hot air at the greenhouse apex. It’s essential to have top vents to allow the hot air out and to encourage this natural flow of air. If you fit automatic vents at the apex then it takes the worry out of remembering to open them in the morning and to close them up at night, it is also an essential addition if you work away a lot or are just not around during the heat of the day in summer. Automatic vents can save your plants and greenhouse contents from overheating.

It’s not really a case of cooling things in the summer as the air outside the greenhouse will be hot anyway, so the only way of reducing the internal temperature is by stopping the suns rays reaching the greenhouse glass with exterior shading or by moving the hot air out as fast as possible. If natural airflow at the top of the greenhouse is not expelling the hot air quick enough then think about introducing a fan. Most electric greenhouse heaters have the option of being an air blower or fan in the summer months, so this may help, but make sure it is positioned so it helps the natural air flow, otherwise it may simply be blowing hot air around the inside of your greenhouse.

Shading helps keep the greenhouse cooler for longer and is most effective fitted to the outside of the greenhouse. However it then becomes subject to the weather and can deteriorate after a few years of wind and rain. You can apply a shading paint to the glass panes but this can look unsightly and can be time consuming to wash off before the winter. Internal shading will help reduce the heat build up in the glasshouse and at the very least you can assist by hanging shade fabric at the glass on the inside or using internal roller blinds, this can protect your plants from direct sunlight. If you have the budget you can opt for automatic vents that are either internal or external and will activate on a timer if required.

Damping down the greenhouse paths, floor and hard surfaces with water, especially at night can cool the greenhouse; the process of evaporation helps reduce the internal temperature and keeps your plants in a better growing environment. Reducing the floor temperature will also start to cool the greenhouse from the floor up. Where possible damp down the paths and floor at night and again in the morning for the best effect.

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