Revision Q & A – June 5th

“How do you define the terms ‘tender’, ‘half-hardy’ and hardy’ as applied to annuals. I’m not sure what the difference is between a tender and a half-hardy annual.”

A hardy annual is a plant which is sown without protection AND completes its life cycle in one growing season. (e.g. Salvia horminum, and Calendula officinalis).

Half-hardy annuals are plants grown as if they had a life span of one growing season, and which are not frost-hardy.

Tender annuals need warm soil to germinate and grow properly, and warm air temperatures to produce the best flower display. They will not tolerate frosts and can be damaged by cool air temperatures above freezing.

“I’m not sure what the difference is between a half-hardy perennial and a tender perennial.”

The basic definition of a perennial is of a plant lasting longer than two years.

Tender perennials are liable to injury or death when subjected to low temperatures. They usually require frost-free conditions, but may survive outdoors in warmer locations. Even if the frost kills off the foliage to ground level, they may re-grow from below ground the following spring.

Half-hardy perennials live for several years, usually flowering from the second season. They are killed by frost, and are often discarded at the end of the season, but they can be overwintered if given frost protection. Examples include: Bellis (daisy), begonia, Pelargonium (geranium) and lobelia.

Hardy perennials such as euphorbia and heuchera are tolerant of some degree of frost. They can give valuable flower and foliage colour through the winter months.

Herbaceous perennials – These plants are non-woody, and often die back to the ground every year, although they may be evergreen. The RHS examiners note “A key phrase defining an herbaceous perennial is ‘non-woody’. Unfortunately this was not included in many answers. Candidates often mentioned dying down to the ground in winter. However, some herbaceous perennials are evergreen.”

“I’m supposed to know about the process of lignification but I can’t find a proper description anywhere?”

Lignification occurs when lignin is deposited in cell walls, usually resulting in the death of the cell.

Lignin is a complex carboydrate polymer (which means it is made up of very long chains) and makes up approximately 25% of the wood in trees. Lignin is water-resistant. It reinforces cell walls, keeping them from collapsing. This is particularly important in the xylem, because the column of water in the hollow xylem cells is under tension (negative pressure) and without the lignin reinforcement the cells would collapse. The Collins Dictionary of Botany notes: Lignin “… is also found in the cell walls of sclerenchyma tissues and vessels, fibres and tracheids at maturity. It increases the strength of such tissues. The process of impregnation of cell walls with lignin is called lignification.”

The Longman Dictionary of Botany notes that during lignification, lignin is “…deposited in the cellulose cell walls of the xylem and sclerenchyma during the process of secondary thickening.”

In Botany James Mauseth notes that: “All plant cells have a thin wall called the primary cell wall. In certain cells that must be unusually strong, the protoplast deposits a secondary cell wall between the primary wall and the plasma membrane. The secondary wall is usually much thicker than the primary wall and is almost always impregnated wih the compound lignin, which makes the wall even stronger than hemicelluloses alone can make it. Lignin resists chemical, fungal, and bacterial attack.”

“When describing the different stages in the life cycle of a plant is it enough to say that a juvenile plant exhibits vegetative, non-flowering growth? And for an adult plant that there is the production of flowers, resulting in fertilisation and production of seed?”

The RHS examiners note that “… juvenility is the period of growth when plants are not yet able to flower.” A key point is that juvenility is an early phase of growth during which flowering cannot be induced by any treatment.

Adulthood can be defined as the stage at which the plant has reached maturity and is able to reproduce.

” ‘Describe how a cultivation pan is formed’. Would it be correct to say that this is caused by cultivating the soil when it is wet, leading to smearing and compaction at cultivation depth? Or is it more to do with always cultivating at the same depth? Would rectifying the problem be digging /subsoiling (If large area)?”

Yes, a cultivation pan will typically form following repeated cultivation under conditions that are too wet. Structural aggregates are destroyed and a compacted, smeared layer is formed. Such pans can form in any type of soil but they are much more common in heavy-textured soils (e.g. Soils with a high clay content) that have a high water content. In some vulnerable types of soil (e.g. clays) a cultivation pan may form simply due to repeated ploughing at the same depth. Rectifying the problem usually means loosening the soil to a depth below the cultivation pan, and would usually involve subsoiling or double-digging.

“I’m not sure what are the benefits and limitations of composted municipal waste as a soil improver. I thought that it was good to recycle garden waste and stop it going into landfill (more environmental factor). I think it would have a high nutrient content. Presumably it could contain weed seeds. I’m not sure if the quality would vary.”

This is about considering a range of factors, and as you suggest it’s important to consider environmental issues too. The RHS note: “Although councils offer green waste collections, the RHS encourages home composting because it does not involve heavy transport, with its associated environmental costs.” In addition, gardeners do not know the source of composted municipal waste and it may well contain weed seeds, pests and diseases and possibly also chemicals (for instance, if the site of origin was treated with pesticides) making it unsuitable for organic growers in particular, but it is an issue of concern for all gardeners. Benefits would be similar to other composts – high organic matter, good nutritional value, and it is also available relatively locally – which is an advantage compared to some composts that are shipped thousands of miles.

Quality may vary depending on the source, but there is now a system of quality standards, so it is worth mentioning this too. Garden Organic note: “Municipal compost that meets the PAS100 quality standard should be fine to use in your greenhouse. To gain PAS100 accreditation, it must go through a monitored composting process that, among other things, ensures that the materials reach significant temperatures for several days, which should kill off most pests and diseases.”

Click here to find out more about PAS 100.

“I’m supposed to know what the importance of lime is to the composting process. However, the RHS website says it’s not normally required!”

Lime is not usually needed unless there is a high content of pine needles or vegetables and fruit, which increase the acidity. It may be used to correct excessive acidity, but in any such answer, also note that it should not be needed if the compost is created with care to include the right balance of ingredients.

” ‘Describe the role of weeds as alternative hosts for plant pathogens’. Apart from stating this fact is there anything else useful to add?!

Good point! I think this is more about understanding the role of weeds. You’re unlikely to be asked this exact thing as a question, it’s more a point of understanding.

You could also note that understanding this aspect of the pathogen’s life cycle (for instance, where a disease such as mosaic virus relies on perennial weeds around the cropping area to over-winter) gives us an important insight into control, as it shows us that by removing the weeds we can remove the virus’s winter home and reduce risk of reinfection of the crop in the following growing season.

“Could you tell me about the lifecycle of the black bean aphid?”

It’s useful to note that most aphid lifecycles are roughly the same. The black bean aphid is not unique from other aphids. However, please see summary below.

Black bean aphid (Aphis fabae)

Egg – nymph – adult

• Overwinters mainly as eggs on spindle, Euonymus europaeus and related plants
• Eggs hatch from late Feb-early April, colonies of nymphs mature on young leaves/shoots.
• Winged forms produced in May/June, and these migrate to a number of summer hosts, including legumes.
• Populations usually peak in July/August.

Life cycle and control: Colonies of nymphs provide food source for ladybirds, lacewings, hoverfly larvae, parasitic wasps.

Click here for more information.