Grapes outside or in a greenhouse?

Harri Poom,  Saare-Tõrvaaugu Gardening Farm

This century, grapes have gained a lot of popularity among both hobby and professional gardeners, and rightfully so.  The development of new varieties right here in Estonia is probably not that far in the future anymore. Today, the most well-known and most grown variety in our home gardens is the Latvian Zilga.

Grape farming is so popular among hobby gardeners that no-one really thinks of it as anything exotic anymore. There are lots of varieties – in Estonia, hundreds of varieties are grown both outdoor and in greenhouses. In the home garden, grapes can be grown in both places, if possible. It is difficult in Northern and Eastern Estonia to find good outdoor varieties, so it would indeed be simpler in those not-so-favourable regions to grow grapes in greenhouses. It does not have to be a heated one; a simple polyethylene tunnel will do.

Do not fear the greenhouse

In South-Estonia, on the islands and on the coast where the summers are warmer, longer or even both, there is no need to worry about growing grapes in the greenhouse. The list of varieties suitable for greenhouses is long and it is possible to grow varieties that beat the the grapes imported from the south not only with their taste and healthiness, but also with the size of the berry and the bunch as well. To be honest, top quality table grapes in Estonia only come from greenhouse complexes. The conditions of outdoor cultivation are too harsh – there’s not enough warmth but too much moisture and the growing time is clearly too short for the better varieties to ripen.

Do not worry about the grape taking control of the greenhouse – each farmer can shape their grapes like a professional if interested and with just a little practice. But if the grape farmer never cuts the grapes back out of greed or stupidity – maybe there are more grapes if I never cut it! – things will indeed get out of hand. It is very important to cut the grapes to get high-quality berries. Also, a grape that’s been uncut for a long time will turn wild and instead of a reasonable amount of big, beautiful and sweet berries, it will often yield a huge amount of poor-coloured and sour ones. Of course, the greenhouse does not need to be glass or plastic; a regular polyethylene one used for growing tomatoes and cucumbers will do just fine. The crest of the grape greenhouse should be at least 2.5 metres – but if it’s more, it’s even better. The grape tunnels in our garden are as much as 4 metres high. They don’t have to be that high, of course, but it won’t hurt, either.

Which varieties should be chosen?

There are so many table grape varieties suitable for growing in greenhouses in Estonia that it is impossible to count them all, and they are so diverse that anyone who likes grapes will find something to their liking.  There are not that many outdoor varieties, but their number is definitely three-digit as well.

The most resilient and undemanding varieties can be found among outdoor ones, of course. The most common and well-known to people are Zilga, Hasanski sladki and Somerset Seedless, but the list is not exhaustive. Over the past years, two super-early outdoor grapes Liiso and Dovga have become popular, both earlier varieties than the three mentioned above. You can see the photos and descriptions of the varieties we have already mentioned, and many others, on the website of our gardening farm.

Of course, the outdoor varieties can be grown in greenhouses as well, and these will ripen much sooner and are sweeter and larger. Liiso and Dovga, for example, ripen in the last few days of July already, a good three weeks before Zilga.

How should you take care of your grapes?

Grapes prefer rich light-textured, moderately humid and only slightly acidic soils with a humus content of 4% at maximum. In soil that is too rich, the plants grow powerfully, but do not carry that many berries. The best planting time is spring or the first half of the summer. In choosing the right time, temperature should be considered: leaf-bearing plants cannot tolerate any cold temperatures and need at least +10 degrees to grow. Of course, transplanting is allowed throughout the summer and in early autumn as well because we can buy young pot plants, so the roots are not damaged when transplanting. Right after transplanting, the plants need to be watered and in the first growing year, the soil should be monitored rather carefully to avoid it being too dry. Depending on the growing site, but on the whole, in any following years, the grapes generally do not need any more watering or (if the soil is dry rather than moist) need less of it. Grapes have large roots that bring the water to them from far and deep. In our own farm, we only water our grapes in the transplantation year, even in the greenhouse.

Regular cutting – key requirement to good crops

Most of the cutting is done in the autumn, a few weeks after the leaves have fallen, but it is important to remove or at least cut back the premature shoots to avoid the plant growing too thick. Grapes ARE NOT CUT in the spring until the shoots are at least 15 cm long – otherwise, there is a risk of excessive “bleeding”, loss of nutrients and overall weakening of the plants.

Grapes are climbers and decent support can be put up in the planting year already – an espalier or a grid – so that the plant would have plenty of room to keep climbing in the next years as well.

The methods for cutting grapes vary greatly – presumed that the cutter knows what he/she is doing. In our farm, we use a pretty simple cutting method that even a beginner can manage. We transplant the grapes at an angle of just 20–30 degrees to the ground because this makes covering the grapes up for the winter, especially the older and larger ones, much easier. In the planting year, we don’t do any cutting in the summer, and cut all grapes planted in the same year back in the autumn so that a small branch with only 3–4 buds would remain. Next spring, we allow two strong shoots to grow and will pinch them back in the summer, removing all premature shoots (that is, the shoots emerging from the leaf axils in the summer). We will also pinch off any flowers, should there be any. After the main cutting in the autumn, two long properly russeted branches will remain. Their length depends on how well the plant has developed, and is usually about a metre or up to one-and-a-half long. In the third year, the first crop is yielded.

Most of the time, there are a lot more flowers than the plant can actually bear. In our farm, we shape the grapes into two long stems and leave no branches on them. This means that from the third year on, we break all shoots that start growing on the branch part planned for stems.

  • The fruiting lateral is short in the first harvesting year, that is, the third growing year, and has only a few buds, but from then on, we leave a long (up to one metre) fruiting lateral on both ends of the stem, and attach it to the lower espalier wire horizontally and route all shoots that start growing on it straight up.
  • In the summer, again, we remove the premature shoots (or remove them behind every next leaf). This means constant work. Of course, we remove all shoots from the stem as well.
  • With the late autumn cutting, we leave one branch sprouted in the same summer on both ends of the stem, which will yield crop the next year, and cut the rest of the branches. And that is how we do it. The stem gets thicker year by year, but the fruiting laterals are annual every year.

A grape may be designed with more than two branches, of course; three and four are completely fine. Glass greenhouses and, in particular, polyethylene greenhouses are colder, whereas plastic retains heat better.

In a warmer (plastic) greenhouse where the plants do not need to be covered up for the winter, the stems can be upright as well to make it more convenient to operate. In longer cold periods, the ground beneath grapes must be covered (with peat or leaves, for example) because the roots are vulnerable: the branches can generally withstand colds of up to -24 degrees without getting damaged (of course, there are more fragile varieties as well), whereas the roots may get damaged at -10 degrees already (measured in soil, not air).