In semi-natural grasslands, mowing leads to the dispersal of species that have viable seeds at the right time. For invasive plant species in grasslands, dispersal by mowing should be avoided, and information on the effect of cutting date on the germination of invasive species is needed. We investigated the germination of seeds of the invasive legume Lupinus polyphyllus Lindl. depending on the cutting date. We measured seed traits associated with successful germination that can be assessed by managers for an improved timing of control measures. To this end, we sampled seeds of L. polyphyllus on six cutting dates and analyzed the germination of these seeds in climate chambers and under ambient weather conditions. We collected information on seed morphology (color/size/hardseededness) for each cutting date to identify seed traits associated with successful germination. Observed germination patterns were highly asynchronous and differed between seeds cut at different dates. Seeds cut early, being green and soft, tended to germinate in autumn. Seeds cut late, being dark and hard, were more prone to germinate the following spring, after winter stratification. This allows the species to utilize germination niches throughout the year, thus indicating a bet-hedging strategy. Seed color and the percentage of hard seeds were good predictors of germination percentage, but not of mean germination time and synchrony. Managers should prevent the species producing black and hard seeds, while cutting plants carrying green and soft seeds is less problematic. Furthermore, germination patterns differed between climate chambers and the common garden, mainly because germination of dormant seeds was lower in climate chambers. More germination experiments under ambient weather conditions should be carried out, as they can give information on the germination dynamics of invasive species.

dormancy, grassland management, lupine, phenology, seed traits

The timing of germination determines which environmental conditions the seedling will experience and thereby influences a variety of plant characteristics (Casas et al. 2012). Consequently, the germination ecology of a species largely decides in which habitats and under which climates it may establish. The introduction of species to new ranges often leads to new germination conditions (Kudoh et al. 2007), and the ability to germinate successfully under a variety of environmental conditions is a characteristic of many successful and widespread invasive species (Baker 1974; Wainwright and Cleland 2013). Whether seeds are viable depends largely on their development stage, which is influenced by the timing of seed set and seed ripening.

In semi-natural grasslands, the mowing date is the environmental factor that most strongly determines the timing of seed release. Furthermore, mowing is a way of seed dispersal for species that have viable seeds at the right time. In most cases, the dispersal of mature seeds after mowing is a desirable process, as it is responsible for sustaining a high plant diversity in semi-natural grasslands (Auffret 2011; Humbert et al. 2012). In other cases, such as weeds or non-native invasive species, dispersal of ripe seeds by mowing is not wanted (Wilson et al. 2009) and shifts in grassland management and the time of cutting may create opportunities for invasives to establish in these ecosystems. Consequently, understanding the germination ecology of invasive plants is essential for their management and control and for limiting their spread to new sites.

Established invasive species are often more challenging to manage than newly arrived species (Simberloff 2003). Ideally, control measures would take place before seed formation, but time windows for adequate management can be short in areas where different conservation goals have to be matched. In the case of species invading mountain grasslands, e.g., mowing of areas critical for the protection of ground-nesting birds has to be postponed until nesting is finished, which means that invasive species may have produced viable seeds by the time of mowing. Consequently, managers are looking for information on the relationships between cutting dates, seed morphology, and seed germinability. Lupinus polyphyllus Lindl. is a widespread perennial legume originating from North-America. It is widely found as an ornamental plant (Fremstad 2010) and commonly used for soil stabilization and soil melioration (Rehfuess et al. 1991). Due to its many uses, the species is naturalized in different regions all over the world, e.g. in Europe (Fremstad 2010; Hejda 2013), New Zealand (Holdaway and Sparrow 2006) and Chile (Meier et al. 2013). Invaded habitats include road verges (Valtonen et al. 2006), riparian terraces (Meier et al. 2013), and mountain grasslands (Klinger et al. 2019). Due to its ability to fix nitrogen, it is considered an ecosystem engineer and may cause unwanted ecosystem effects (Hiltbrunner et al. 2014). In invaded habitats, L. polyphyllus is capable of overgrowing and shading the underlying vegetation and may cause a considerable decline in the richness of small species (Thiele et al. 2010; Hiltbrunner et al. 2014), while promoting the spread of tall-growing, nitrogen-demanding vegetation (Otte and Maul 2005). Meadows invaded by this species provide hay of low fodder quality, because of its high water-content and the presence of alkaloids in L. polyphyllus (Hensgen and Wachendorf 2016).

Despite the importance of seed ecology for the spread and establishment of species, there is often insufficient knowledge concerning germination and ripening characteristics of invasive species (Gallinat et al. 2018). The capability of seeds to after-ripe and germinate, which depends on the interaction between phenology and cutting date, may have important implications for the management of invasive species in grasslands. Therefore, we investigated the germination of the invasive legume L. polyphyllus in relation to the cutting date. Over the course of the vegetation period, i.e., weekly from the beginning to the end of fruiting, we sampled seeds from five locations invaded by L. polyphyllus. We combined two experiments to investigate the germination of L. polyphyllus: A common garden experiment to analyze the germination patterns under ambient weather conditions and a climate chamber experiment under standardized conditions. We aim to provide management recommendations based on seed traits such as seed color and hardseededness that may help to decide when fruiting lupine stands should be cut and when plant material has to be removed from the sites after mowing.

Specifically, our research hypotheses were:

1) The germination ability of L. polyphyllus seeds increases with later cutting date. Consequently, we expect a higher germination percentage, a shorter mean germination time, and a higher synchrony of germination with later cutting date.

2) Seed traits such as seed size, seed color, and the percentage of hard seeds provide reliable information about the germination ability of seeds sampled at different dates. We expect larger seeds, seeds with darker color and harder seeds to show higher germination percentage, shorter mean germination time and higher synchrony compared to small, green, and soft seeds.

During the sampling period, seed color became darker (changing from green via dark green and brown, to black) and the proportion of hard seeds increased gradually. Mean seed size ranged from 3.9 mm (date six, August 11^th) to 6.4 mm (date three, July 21^st). It increased during the first three weeks of cutting and then decreased thereafter as seeds became drier. Seed color and the proportion of hard seeds were correlated, as hard seeds usually were darker than soft seeds. There were no differences in the total germination percentages between different sampling locations, although the germination peaks shifted by up to two weeks between different locations.

In climate chambers, 16.3% of all collected lupine seeds germinated (Fig. 1a, b). Germination percentage was lowest after the first cutting date (July 7^th, 8.6%) and increased until the third date (July 21^st) where it peaked at 26% (Fig. 1a). Afterwards, we observed a significant decrease from week three (July 21^st) to four (July 28^th; to 13.4%; Table 1). Mean germination time was 114 days and varied from 3 days to 303 days in climate chambers (Fig. 1c, d), with seeds collected on the first date having the longest mean germination time (141 d; Fig. 2d). Mean germination time decreased until week three (98 d), then increased again and had its overall minimum in week six (74 d). Synchrony of germination was quite low with an average of 0.08 over all cutting dates (Fig. 1e, f).

Figure 1.

The effect of the factors cutting date (weekly from July 7^th to August 11^th) and seed color on germination percentage (a, b), mean germination time (c, d), and synchrony of germination (e, f) in seeds stored in climate chambers averaged over the two temperature regimes. Bars show mean values ± standard errors.

In the common garden, 51.7% of seeds germinated and mean germination time was 153.6 days (Fig. 2). Thus, seeds in the common garden germinated to a higher degree compared to seeds in climate chambers, but slower. Germination percentages in the common garden were lowest in seeds sampled during the first two weeks (17.0% on July 7^th and 30.6% on 14^th), reached the highest level in week three (63.2% on July 21^st) and stayed high afterwards (Fig. 2a, Table 1). In the common garden, mean germination time was similar for all cutting dates and averaged 153.6 days. Synchrony of germination in the common garden was quite low with an average of 0.12 over all treatments and on all cutting dates (Fig. 2e, f).

Figure 2.

The effect of the factors cutting date (weekly from July 7^th to August 11^th) and seed color on germination percentage (a, b), mean germination time (c, d), and synchrony of germination (e, f) in seeds stored under ambient weather conditions. Bars show mean values ± standard errors.

There were significant differences in germination percentages between seeds of different color (Figs 1b, 2b, Table 2). In climate chambers, dark green seeds showed the highest germination while in the common garden, germination percentages increased steadily as seeds darkened (Figs 1b, 2b). In climate chambers, germination percentage peaked when 60% of collected seeds were hard and decreased when the amount of hard seeds was lower or higher while in the common garden, germination percentage increased continuously with the amount of hard seeds. In both experiments, seeds of different color had relatively similar germination times with black (99 d) and dark green (109 d) seeds germinating most rapidly in climate chambers (Fig. 1d). In the common garden, there were no significant differences in mean germination time between seeds of different colors (Fig. 2d).

While in climate chambers, germination peaked early and decreased afterwards (Fig. 3), two peaks (in autumn and spring) characterized germination in the common garden (Fig. 4). There were no significant differences between colors in climate chamber, while synchrony in the common garden increased slightly with the increase in the percentage of hard seeds.

Figure 3.

Germination patterns of L. polyphyllus in climate chambers conditions (15/5 °C and 20/10 °C diurnally fluctuating temperatures) sampled weekly on six cutting dates (July 7^th to August 11^th) after seed set.

Figure 4.

Germination patterns of L. polyphyllus under ambient weather conditions sampled weekly on six cutting dates (July 7^th to August 11^th) after seed set.

Germination percentage (in both experiments) and mean germination time (only in climate chambers) responded significantly to cutting date, while there was no effect of the different temperature regimes. For germination percentage and mean germination time, the best explanatory models (see Suppl. material 1: Model Tables) each contained solely one fixed factor, mainly due to high correlations between explaining factors. Germination percentage in climate chambers was best explained by seed color and showed highly significant differences between colors (R² = 0.15). Germination percentage in the common garden was well explained by both seed color (R² = 0.58) and proportion of hard seeds. For mean germination time, the best explaining factors were either color or proportion of hard seeds, while both models performed poorly overall. Synchrony was not affected significantly by any factor and there was no model of significant explanatory value.

The germination patterns of the invasive legume L. polyphyllus differed between different cutting dates, partially confirming our first hypothesis. Seeds collected early, while being green and soft, germinated to a lower degree and more slowly compared to seeds collected later. While seeds of early-cut L. polyphyllus plants germinated in autumn, seeds of late-cut plants were more prone to germinate in spring. This relationship may be associated with their progression through different phases of seed development. During morphogenesis the embryo develops, then during maturation, storage compounds are synthesized in the growing endosperm and thereafter, seeds may go through a phase of desiccation, in which they dry and eventually enter dormancy (Angelovici et al. 2010). Consequently, the different cutting dates of our experiment covered the phases of maturation and desiccation. Until late July, L. polyphyllus seeds were in the phase of maturation. Afterwards (end-July to mid-August), seeds were in the desiccation phase. Although dormancy per se was not tested in our study, the observed germination patterns and differences between climate chambers and the common garden strongly indicate that seeds from late cut L. polyphyllus plants expressed dormancy, which is also supported by our observation that seeds decreased in size and became harder. Physical dormancy is common in legumes (Russi et al. 1992a), but whether an individual plant produces dormant seeds at a given point in time depends on a variety of factors, such as temperature and moisture conditions during seed ripening (Masaka and Yamada 2009; Bolingue et al. 2010; D’hondt et al. 2010). Thus, the expression of dormancy can vary strongly in legume seeds, even within plants of the same population (D’hondt et al. 2010), which may consequently lead to asynchronous germination patterns.

Despite pronounced peaks of germination in autumn and spring, germination of L. polyphyllus seeds was highly asynchronous. In both experiments and under all cutting dates, some seeds germinated over the whole duration of the experiments, over 300 days. The timing of germination determines which environmental conditions the seedling will experience and may influence plant characteristics, such as growth and reproduction (Donohue 2002; Casas et al. 2012). The timing of germination itself may be influenced by plant life-history traits, e.g. the phenology of flowering, seed maturation, and seed dispersal (Galloway 2001; Donohue 2002). Variations in germination depending on the time of seed collection have been observed by other authors (e.g., Greipsson and El-Mayas 2003; Samarah 2005; El-Keblawy and Al-Rawai 2006; Brobäck 2015), but there is little information on the long-term germination patterns of species and seasonal effects that are associated with this factor. In invasive species, asynchronous germination can lead to the exploitation of open germination niches throughout the year, which might contribute to their invasion success (Wolkovich and Cleland 2011; Gioria et al. 2016). In the case of L. polyphyllus, this effect may be amplified by its high seed production (Volz 2003), its long-lasting flowering, by its ability to resprout and produce seeds after early cutting (Brobäck 2015), and by the observation that the ballistic seed dispersal of the species takes place over many weeks if stands are left untouched (Klinger et al., unpublished data). The observed germination patterns of L. polyphyllus thus suggest a bet-hedging strategy (Cohen 1966), which may partly explain its invasion success and its capability to colonize many different habitats.

Our second hypothesis can be verified, as seed color and the percentage of hard seeds were good predictors of germination percentage and give information on the germination patterns that can be expected. Soft and green seeds germinated to the lowest degree and in autumn. However, germination percentages of these seed batches were relatively high, given their early developmental phase. High germination rates in immature seeds have been found in some legumes, e.g., in Lotus and Scorpiurus (Cristaudo et al. 2008), and Vicia (Samarah 2005), but germination failed in others, such as in green seeds of Lupinus nootkatensis (Greipsson and El-Mayas 2003). Black and hard seeds germinated to a high degree and in spring. In temperate climates, seedlings germinating in autumn face harsh environmental conditions during winter combined with low competition, while spring germination is associated with more favorable environmental conditions, but higher competition (Masuda and Washitani 1992). Since soft and green seeds mostly germinated in autumn, the winter survival of the emerging seedlings may be low, as L. polyphyllus seedlings seem to be sensitive to freezing and showed high mortality when exposed to –10 °C (Arfin‐Khan et al. 2018). Furthermore, unripe seeds of roadside L. polyphyllus stands in Sweden were prone to mold infection that led to very low germination rates (Brobäck 2015). The last cutting date represents the state in which seeds are shed by the plant. Both ballistic seed dispersal as well as the expression of physical dormancy go along with the drying of the pods and the seed coat. Black and hard seeds are more prone to germinate in spring and may thus have higher survival rates compared to green seeds. Furthermore, as L. polyphyllus follows a c-strategy (Grime et al. 1988), it may be able to cope with higher competition in spring, especially in habitats with weak competitors, such as semi-natural grasslands. Additionally, water impermeable/hard seeds are more prone to being carried over into the seed bank (Russi et al. 1992b) or dispersed via endozoochory (Otte et al. 2002; D’hondt and Hoffmann 2011). Although L. polyphyllus may not have invaded the seed bank of meadows in our study region yet (Ludewig et al., unpublished data), a carry-over of seeds should be avoided, as it makes invasive species management lengthier and more expensive. Consequently, managers should target plants that still have green and soft seeds, which can be considered less problematic despite germination percentages being relatively high.

Germination patterns differed between climate chambers and the common garden, particularly after seeds darkened and became harder. Overall, germination percentages in the climate chamber experiment (ca. 16%) were similar to the emergence rates found by Sõber and Ramula (2013) (21.5%), but relatively low compared to other studies on L. polyphyllus (Elliott et al. 2011; Arfin-Khan et al. 2018; over 60%). We suggest that this is at least partly due to the fact that seeds were not scarified and that dormancy was probably not broken by imbibition in the climate chamber experiment. This is also supported by the results of the common garden experiment, in which germination percentages were considerably higher than under laboratory conditions, mainly due to a second germination peak in spring after winter-stratification in situ. However, germination of L. polyphyllus only slightly increased when seeds were pre-treated by cold in another study (Elliott et al. 2011). Our results show that, while laboratory experiments give valuable information on the environmental factors influencing germination, the germination patterns observed under artificial conditions may diverge from germination dynamics under ambient weather conditions (Hölzel and Otte 2004). A better understanding of invasive species germination under natural conditions is necessary, as it can potentially reveal windows of opportunity for invasive species management. We thus recommend to complement germination experiments in climate chambers with common garden or field experiments.

Seeds of L. polyphyllus are capable of after-ripening and germinating even if plants are cut while most seeds are still green and soft. Germination capability increased strongly during the first weeks after seed set with a maximum when most seeds were brown to black and not fully hardened. Therefore, L. polyphyllus stands should be cut before seed set, if possible. If this is not feasible due to different limitations, we recommend cutting while plants carry green and soft seeds. When stands with black and hard seeds are cut, the plant material should be removed immediately to reduce propagule pressure on site.

We would like to thank Dr. Reinhard Stock, Dr. Volker Wachendörfer, and Dr. Franz-Peter Heidenreich for continuous interests and suggestions as well as constructive support of our study. Furthermore, we thank Torsten Kirchner (Wildlandstiftung Bavaria), Michael Geier and Tobias Gerlach (Bavarian Administration of the Biosphere Reserve Rhön), as well as Torsten Raab (Hessian Administration of the Biosphere Reserve Rhön). We greatly thank Josef Scholz vom Hofe for assistance with propagule and data collection both in the field and in the lab, Sabrina Rothen for data preparation, Sarah Harvolk-Schöning for statistical support, and Melanie Schindler for valuable comments on the final version of the manuscript. We are grateful to two anonymous referees for their insightful comments on an earlier version of the manuscript.